RESEARCH INTERESTS

My research activities bridge the areas of inorganic chemistry, applied physics and materials science of semiconductor and refractory materials. The design, preparation and applications of novel solid state and molecular systems are particularly emphasized. Current thrusts include: (a) synthesis of purpose-built, main group inorganic hydrides with tailored reactivities and stoichiometries, enabling formation of functional material architectures that cannot be obtained by conventional routes, (b) growth of silicon-based photonic materials for the development of prototype photodetectors, modulators, and multijunction photovoltaic devices, (c) integration of dissimilar materials with Si technologies (including III-V and II-VI compounds for monolithic integration) via epitaxy driven synthesis methods, (d) advanced wide band gap semiconductor materials for breakthrough applications in solar energy, solid state lighting and optoelectronics semiconductors, (e) solid-state inorganic chemistry based on light elements (refractory carbides, nitrides, borides and C-N frameworks).

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Synthesis of materials and devices of Si-Ge-Sn semiconductors for applications in group IV, Silicon based optoelectronic technologies

Overview:

DeviceFigure
Photograph of Ge1-ySny photodiode devices.

The combination of expertise in inorganic chemistry, solid state chemistry and applied physics has allowed our group to innovate complicated and technologically advanced and exceedingly useful materials in the Si-Ge-Sn system using the practical application of a fundamental knowledge in main group hydride chemistry, epitaxy driven synthesis on suitable templates as well as advanced optical characterizations and device fabrication techniques.

Highlights: Our group at ASU has made pioneering contributions in establishing Ge1-ySny as the first direct gap semiconductor among the group IV materials and a practical and widely accepted semiconductor system with widespread applications in both high speed electronics and infrared photonics. In 2002 our group developed the first CVD synthesis of Ge1-ySny using reactions of Ge2H6 and SnD4 molecules that were designed by us specifically for this application. This research yielded crystalline alloys at CMOS compatible conditions with Sn contents up to 18% directly on Si wafers. This achievement propelled us to then map the band structure, demonstrate tunable photoluminescence (PL) for the first time in 2009, and develop doping protocols that enabled the fabrication of the first generation light emitting diodes (LEDs) and p-i-n photodetectors covering all telecommunications bands in the following years 2010-2011. In concert with Ge1-ySny in 2003 we also synthesized an entirely new class of Ge1-x-ySixSny ternary alloy analogs. We specifically developed a subclass of these materials with composition Ge1-x(Si4Sn)x exhibiting independently tunable band structure and lattice constant for the first time in group IV materials in 2009. This eventually led to our design and fabrication of high performance photodiodes with 1 eV optical band gap in 2011 for possible applications in multifunction photovoltaics.

Our research continued to advance the field and subsequently in 2012, we successfully introduced next generation hydride sources Ge3H8 and Ge4H10 whose enhanced reactivity allowed facile incorporation of greater Sn fractions into Ge and Si1-xGex. In addition the replacement of Si wafers with Ge buffers as growth platforms led to better crystallinity enabling a systematic development of materials and devices, exhibiting tunable PL and electroluminescence (EL) between 2000 nm and 2800 nm (the figure above is a photograph of PIN diode devices). We established that Ge1-ySny behaves as a direct gap semiconductor using photoluminescence studies in 2013-2014. The onset of indirect to direct gap transition near 7% corresponds to the lowest Sn level observed to date and is likely due to high n-type doping, highlighting the ability of our method to produce materials not accessible by competing methods. The ability to dope deliberately over a wide range enabled fabrication of next-generation LEDs and p-n tunneling light sources and accelerated refinement of their optoelectronic performance in 2014-2015 (the figure below shows a prototype LED structure).

Prototype LED
Prototype LED structure

In 2013-2015 we developed the next generation Ge1-x-ySixSny alloys whose electronic properties mimicked those of binary Ge1-ySny analogs including direct gaps and tunable absorption edges extending from near IR into the mid IR. The direct gap behavior combined with the enhanced thermal stability makes these alloys a better alternative to Ge1-ySny for direct gap group IV devices. In 2019 we achieved a further major breakthrough by developing a new series of alloys possessing compositions 20-35 % Sn fully integrated on Si (model of Ge0.67Sn0.33 alloy is shown below). This recent study has overcome prior compositional barriers for Ge1-ySny semiconductors and has advanced the technology to new levels spanning a significant swath of the wide IR portending application in the MWIR and thermal infrared (MWIR/LWIR) spectral ranges. At this stage we can make the case that Ge-Si-Sn alloys represent bona fide IR semiconductors among those of conventional group IV materials and we are currently working to significantly advance the state of the art.

Ge0.67
Structural model of 33% Sn alloy synthesized at 245 oC (Chem. Mater. 31(23),9831-9842 (2019)

 

The Si-Ge-Sn system prior accomplishments and future directions:

Fig1_
Fig 1: Model of a Ge1-ySny alloy produced on Si(100) by CVD reactions of Ge2H6 and SnD4

The Chemistry: The substitution of Sn into the Ge lattice produces random Ge1-ySny alloys. These incorporate chemically compatible constituent atoms but with very large size mismatches that makes them thermodynamically metastable and thus difficult to synthesize using conventional routes. The structure of these materials can be described with an average diamond cubic lattice as shown by the model (Fig. 1) in which the blue lines represent the Ge lattice and the grey spheres correspond to substitutional Sn atoms.

Fig2_
Fig. 2: The substitution of Sn into the Ge lowers the direct gap faster than the indirect inducing a cross over from indirect to direct gap semiconductor suitable for optical applications. The schematic diagram of the band structure for Ge1-ySny as a function of concentration, illustrates the direct band gap closing from SWIR to LWIR and then becoming negative with increasing Sn content.This is consistent with experimental observations.

The Technological Importance: Ge1-ySny alloys have been attracting increasing attention over the past two decades as a result of its compatibility with Si technologies and their direct band gap behavior in the near infrared (NIR) region of the spectrum. Pure germanium is an indirect gap semiconductor but the separation between its fundamental band gap and the lowest direct gap is only 130 meV. This difference is reduced by alloying with Sn, inducing a direct-indirect crossover for y ~ 6-9% Sn. At these concentrations the system is well beyond the solubility limit of Sn into Ge but still within the dilute alloy regime, in which Sn atoms are mostly isolated within a Ge matrix. Further increases in the Sn concentration reduce the energy band gap even further as is illustrated schematically in Fig. 2, which shows that the direct gap closes from short-wavelength infrared (SWIR) to mid-wavelength infrared (MWIR) and then becomes negative with increasing Sn content. The near band gap band dispersion becomes nearly linear in the Kane limit of vanishing direct gap.

Our Advances and Breakthroughs: Ge1-ySny is the first direct gap semiconductor among the ubiquitous group IV materials Si, Ge and Si1-xGex. Our group at ASU has made pioneering contributions in establishing Ge1-ySny as a practical and widely accepted semiconductor system with widespread applications in both electronics and photonics. In 2002 our group developed the first chemical vapor deposition synthesis using CVD reactions of purposely designed molecules which yielded device grade alloys at CMOS compatible conditions. The initial syntheses were conducted in at that time by reactions of SnD4 and Ge2H6 at 330-400oC (Fig 1), yielding the first generation alloys with Sn contents up to 18% directly on Si wafers. This achievement allowed us to map the band structure and demonstrate direct gap photoluminescence (PL) for the first time in 2008 from specially selected samples created by our method from 2002. We then developed n- and p-type doping protocols that enabled the fabrication of prototype devices such as p-i-n photodetectors and light emitting diodes (LEDs) for the first time. These devices exhibited tunable absorption and emission wavelengths beyond that of Ge at 1550 nm down to 2000 nm into the near IR with enhanced performance relative to Ge due to the higher absorption coefficients of the alloys.

Fig3_
Fig 3: Direct gap photoluminescence vs. temperature of n-type Ge0.93Sn0.07 showing direct gap behavior as evidenced by the sharp increase in PL intensity with decreasing temperatures from 300K-10K.

Our research continued to advance and subsequently in 2010, we successfully replaced Ge2H6 with the more reactive Ge3H8 thus lowering the deposition temperature (330-280oC) required and therefore allowing the incorporation of greater Sn fractions into Ge. In addition the replacement of Si wafers with Ge buffers as growth platforms led to better crystallinity. The Ge3H8/SnD4 on Ge method enabled a systematic development of materials and devices, exhibiting tunable PL and electroluminescence (EL) between 2000 nm and 2800 nm. Typical PL spectra are shown in Fig. 3 for a 7% Sn alloy doped n-type at 3.1x1019 phosphorus carriers/cm3. The material behaves as a direct gap semiconductor, as evidenced by the increase in PL intensity with decreasing temperatures from 300K-10K. The 7% indirect to direct gap transition corresponds to the lowest Sn level observed to date and is likely due to high doping, highlighting the ability of our method to produce materials not accessible by competing methods.

Fig4_
Fig.4: Electroluminescence from n-i-p hetero- and homo-structure diodes up to 12 % Sn. Elimination of defected Ge/GeSn interfaces in the n-i-p stack leads to superior emission efficiency.

The ability to dope deliberately over a wide range enabled fabrication of next-generation LEDs and accelerated refinement of their optoelectronic performance. Fig. 4 shows that hetero-structure LED designs with one defected interface exhibit strong, tunable electroluminescence and a red shift in emission energy and an increase in peak intensity with increasing Sn content. Homo-structure analogs comprising n-i-p layers with identical Sn contents (~ 7% Sn) and defect free interfaces produce much higher efficiency devices.

Synthesis and devices of Ge1-x- ySixSny semiconductors

Fig5_
Fig. 5: Equilibrium isomeric mixtures of Si4H10 and Ge4H10 react with SnD4 to produce Ge1-x-ySixSny photodiodes with average diamond structure and defect free microstructure. The geometry of device is shown.

The synthetic strategies that enabled the above advances of Ge1-ySny on Si were subsequently applied by our group to produce entirely new Ge1-x-ySixSny ternary alloys in 2003. This is the first group IV material system with a widely tunable composition space enabling new functionalities in group-IV materials, including realization of direct-gap systems and independent tunability of band structure and lattice dimensions. Our work in this area has led to several, well-defined families of materials with specific electronic properties identified by theory. A classic example is the Ge1-x-ySixSny (x=4y) ternaries with lattice constants identical to Ge. Single layers were grown on Ge-buffered Si and used to demonstrate band gap and lattice constant decoupling for the first time in group IV materials. Subsequently p-i-n devices were fabricated defect-free on Ge wafers. This is shown in Fig. 5 which illustrates Si4H10, Ge4H10 and SnD4 molecules reacting stoichiometrically to produce layers with diamond like tetrahedral structures devoid of defects as required to for the creation of working devices. Photodetectors were fabricated exhibiting record low dark currents, tunable absorption edges and high collection efficiencies indicating applications in Si-Photonics including photovoltaics.

Fig6and7_
Fig. 6(on the left): EQE vs wavelength plot of Ge1-x-ySixSny /Ge(100) p-i-n device with 1 eV direct gap and collection efficiency 0.75. Fig. 7(on the right: Plots of Edirect and Eindirect bandgaps vs. Si and Sn show a direct gap crossover at 0.45 eV for Ge1-x-ySixSny.

Photocurrent plots were used to determine direct gaps above and below 1 eV vs. composition up to ~ 40% Si. Figure 6 shows absortion curves of a representative Ge1-x-ySixSny (x=4y) on Ge device demonstrating high quantum efficiencies as a result of the absence of crystalline defects. Next generation Ge1-x-ySixSny alloys were developed in 2013 by our group. These incorporated a fixed small amount of Si<5% and varying amounts of Sn up to at least y~0.15. The electronic properties were similar to those of binary Ge1-ySny alloys including direct gaps and tunable absorption edges extending from near IR into the mid IR. The direct gap behavior of this system combined with the enhanced thermal stability makes these alloys a potentially better alternative to Ge1-ySny for direct gap group IV devices. Light emitting and absorbing devices including photodetectors and LEDs have been fabricated. Furthermore, the direct and indirect energy gaps have been determined and are compared Fig. 7 as a function of Si and Sn content with those of the prior Ge1-x-ySixSny (x=4y) analogs. The graph shows that the two systems combined provide band gap engineering over a wide range. Most importantly a cross over to direct gap semiconductor is observed at ~ 0.45 eV. Recent studies by other groups have corroborated the formation of Ge1-x-ySixSny, but a systematic tuning of the composition is rarely demonstrated. Our approach to these systems has focused on developing strategies that actually enable systematic tuning of the compositions and we continue to pursue this path toward further development of Ge1-x-ySixSny from fundamental and practical perspectives. At this stage we can make the case that Ge1-x-ySixSny represents a bona fide IR semiconductor in its own right and we are currently working to significantly advance the state of the art.

Fig8and9_
Fig. 8(on the left): Sn-containing semiconductors in the literature up to 2018. Dotted line represents compositions lattice-matching Ge. The red bar between 18-36% Sn marks newly prepared mid-IR compositions. Fig. 9(on the right): The solid lines represent the experimental absorption edges from selected Ge1-ySny alloys. The compositions are indicated at the bottom of the traces.

The IMPACT: Following the above breakthroughs, many groups worldwide started programs and have reported fundamental advances in the synthesis and understanding of these metastable alloys and have demonstrated advanced devices such as photodetectors, avalanche photodiodes and lasers for potential integration of Si based electronics with optical components. The phase diagram in Fig. 8 shows the most common compositions reported in recent years including those of binary Ge1-ySny alloys with 0<y<0.20 while ternary Ge1-x-ySixSny analogs have a virtually unexplored two-dimensional compositional space, except partially along the line corresponding to our Ge1-x-ySixSny (x=4y) system lattice-matching with Ge described above.

The Road Ahead:

(a)Synthesis of mid infrared group IV (Si-Ge-Sn) semiconductors in the 3-9 m range and beyond

A significant recent thrust in the development of IR materials for long wavelength device applications is to extend the optical range of Ge1-ySny devices beyond the state-of-the-art 1.55 m - 3.0 m window into the mid IR and Tera-Hz range between 3-12m. This requires synthesis of new and metastable alloys with ultra-high Sn contents that cannot be produced by current methods. We achieved a major breakthrough in 2019 by developing specifically designed gas phase chemistries that yielded a new series of alloys possessing the highly sought compositions between 20-35 % Sn fully integrated on Si and designated as new alloys in the Fig 8. This recent study has overcome prior compositional barriers for Ge1-ySny semiconductors and has advanced the technology to new levels spanning a significant swath of the wide IR range. The band gaps of these materials are measured to be as low as 0.15 eV (31-32 %Sn), which allows them to cover the mid-IR optical range past 8 m. The high thermal stability of these material (250-300oC) suggests practical device fabrication is within reach, making them a technological alternative to Hg1-xCdxTe in the MWIR and thermal infrared (MWIR/LWIR) spectral ranges. Figure 9, shows distinct absorption edges for a series of alloys with Sn contents above 13 %. The edges shift monotonically to lower energies as the Sn concentration is increased, reaching 8 m for ~ 30 % Sn. This indicates that systematic tuning of the optical properties has been achieved. Further reduction of the band gap energy has been measured for newer samples with 30-35% Sn contents not shown here. The results collectively indicate a strong compositional dependence of the bowing parameter.

Fig11_
Fig. 10: Atomic resolution image of 27% Sn alloys integrated on silicon.

The main difference between Ge1-ySn y (y 0.35) and the more diluted alloys (y < ~0.2) featured in prior studies is the predicted presence of a significant fraction of Sn-Sn bonds, which could have a profound impact on the structural and electronic properties in view of the large size mismatch between Ge and Sn. However, X-ray measurements of the lattice parameter over the entire 0-35% Sn compositional range show a linear dependence of the cubic lattice parameter consistent with Vegards law. Calculated, structural properties are also consistent with these experimental observations of diamond cubic random alloys exhibiting Vegard behavior. Structural models (not shown) emphasize highly dense arrangement of Sn atoms in the Ge diamond structure. Figure 11 shows the microstructure of a representative 27% Sn alloy. The atomic resolution image corroborates the single crystal character of the material. The Ge1-ySny crystal is homogeneous and diamond like throughout at the nanoscale and exhibits a flawless epitaxial registry with the Si wafer.

(b)Technical Challenges

Fig_HEA
Fig. 11: SiGeSn ternary system showing the region of high entropy alloys (HEAs) in the middle of the diagram.[J.Appl.Phys.126,225703(2019)]

The 3-12 m mid IR range corresponds to an atmospheric window of importance for IR detection capabilities of interest to civilian and military applications. The newly synthesized Ge1-ySny alloys with 20-35 % Sn contents overlap with HgCdTe for these applications, and offer potential advantages in terms of cost, weight, ease of Si integration, and the option of using Si technology to create array detector. Ongoing work is focused on further synthesis and optimization of materials quality and development of doping protocols en-route to devices. Band gap determinations using spectroscopic methods are in progress indicating that the energy gap decreases and the band gap shows a cubic deviation from the standard quadratic alloy expression. The cubic term may dramatically impact the ability of the alloys to cover the long-wavelength (8-12 m) mid-IR atmospheric window. Our synthetic efforts are aimed to produce new materials with higher Sn contents and progressively narrower bandgaps approaching the expected zero limit (we have recent evidence of a vanishing band gap near 20 microns with increasing Sn content). For this purpose new synthesis procedures are implemented using pure Sn hydride sources with tunable reactivities.

(c) Exploratory synthesis of high entropy alloys in the Ge1-x-ySixSny system

One recent thrust in this area is the synthesis of previously unexplored high Sn/high Si content Ge1-x-ySixSny alloys with Si, Ge and Sn compositions approaching equimolar amounts. Such materials have been recently predicted to belong to the general class of high entropy alloys due to the random local bonding afforded by the similar electrohegativities of the isovalent atoms (Zhuang et al. J. Appl. Phys. 126, 225703 (2019). These considerations are purported to increase the configurational entropy making these materials a promising target for synthesis of low cost mid IR systems compatible with Si processing.

 

Doping technologies for applications in group IV semiconductor optoelectronics

The electrical properties of semiconductors can be tuned by doping the materials with donors or acceptor atoms. The technological importance of this tunability has motivated decades-long effort to elucidate the incorporation and electrical activation of the dopant atoms. Materials containing high dopant levels are often synthesized by ion implantation of pristine semiconductors followed by annealing. This approach is not ideally suited for fundamental studies as well practical applications in devices due to residual damage, non-uniform doping profiles, and partial activation of the dopants. The drawbacks associated with implantation have been recently circumvented by in situ doping techniques.

Fig12_
Fig. 1: Molecular approaches to in situ doping by CVD using a select group of P, As, Sb, B, Ga hydrides with complementary deposition properties

We have pioneered the development of doping methods for group IV Si-Ge-Sn semiconductors using specifically designed molecular precursors with reactivities compatible to the low-temperature CVD synthesis at which dopant diffusion is minimized and the materials structural integrity is preserved. The graphic in Figure 1 shows that tunable n-and p-type doping is achieved at conditions compatible with Ge1-ySn y synthesis using P, As, Sb, B, Ga hydrides with complementary deposition properties. More recently gallium hydrides were introduced as an alternative to B2H6 for long-wavelength (mid-infrared) device processing requiring ultra-low temperatures. The samples and devices are typically grown on Ge-buffered silicon platforms via either a single wafer tool (Fig. 1 lower left) or a multi-wafer CVD batch reactor (Fig. 1 lower right). The availability of materials doped in situ at ultra-high levels via the above method has made it possible to achieve record-low resistivities and discover new physics, including universal laws for the dependence of lattice parameters on doping content or the observation of phase-filling singularities in the optical dielectric function. In addition, uniform dopant profiles are obtained and sharp well defined interfaces are formed promoting fabrication of working p-i-n diode devices such as light emitting diodes (LEDs) and photodetectors. As an example, Figure 2 below shows an ultra-low temperature method for in situ doping utilizing a custom hydride P(SiH3)3. This compound is structurally and chemically compatible with the tetrahedral host lattice promoting intact incorporation of the molecular PSi3 molecular cores. Activation of the entire dopant concentration is achieved via this method. Figure 3 below shows our process to produce Ge:Sb materials

Fig13and14_
Fig. 2(left): Schematic of the molecules (top) and doped Ge film structure (bottom). A PSi3 unit embedded within the bulk Ge host lattice is shown. Fig. 3(right):Reactions of Ge3H8 and SbH3 to produce hyper-doped Ge:Sb with fully substitutional Sb contents and ultralow resistivities

Our current program in this area involves two parallel efforts. First, we attempt to optimize n-type molecular sources of P, As, and Sb and p-type sources of B and Ga that we have already demonstrated in Ge, Si, Ge1-ySny, and Ge1-x-ySixSn group IV materials. The second effort concentrates on the development of the new doping sources containing Bi. The possibility of n-type doping with Bi would provide a unique opportunity to better understand the chemical trends apparent in multi-donor studies involving S, P, As, and Sb. Furthermore Si:Bi represents an intriguing alternative to Si:P for quantum computing applications. In summary, we continue to explore the ultimate limits of carrier concentrations that can be achieved with our present dopant portfolio as well as extend this portfolio by exploring additional precursors technologies.

 

Mono-crystalline, silicon-like (III-V)-(IV)3  and (III-V)y -IV5-2y semiconductors

(III: B, Al, Ga, In)  (V: N, P, As, Sb)  (IV: Si, Ge)

We explore synthesis of a new class of hybrid tetrahedral semiconductors consisting of alloys of group-IV and III-V compounds that are inaccessible using traditional methods.  The work is based on a recently discovered methodology in our labs  that combines III-V and group-IV materials in a manner that eliminates phase segregation issues that until now prevented the development of these materials for applications in optoelectronics. The approach involves use of purposely assembled molecular building blocks that incorporate fully pre-formed tetrahedral units to create single phase monocrystalline epilayers via epitaxy driven mechanisms.  The project includes the rational design and development of main-group hydrides with precisely tailored structures and stoichiometries at the nanoscale that enable the generalization of the approach. These precursors are then used for crystal growth of new semiconductors with natural formula (III-V)(IV)3, on silicon, germanium and other industrially relevant platforms. The structural and optical properties of the resultant crystals are fully explored.  Ab initio simulations are used to guide the selection of target systems with potentially attractive properties.

AlPSi3

While this new method was initially introduced to grow new mono-crystalline compounds such as Si3AlP (see figure above), Si3AlAs, and Si3Al(As1-xPx) as well as corresponding alloys Si3Al(As1-xNx), Si3Al(P1-xNx), Al(As,P,N)ySi5-2y and (InP)yGe5-2y, it can be generalized to include most group IV and III-V elements. We have demonstrated that judicious alloying of the group V sublattice allows tuning of structural and optical properties, including perfect lattice matching to silicon, in the general Al(As,P,N)ySi5-2y class of compounds. The synthesized materials may have applications in various fields of optoelectronics, including direct-gap laser materials on Si and semiconductors with widely tunable band gaps for high efficiency photovoltaics.This is a project for students who are interested in hands-on experiments involving epitaxy driven synthesis of new optical semiconductors and characterization of structure/composition/ property relationships.  Methods of synthesis include gas source molecular beam epitaxy and ultra-high vacuum chemical vapor deposition.  No prior experience is necessary, but epitaxial growth and characterization of device quality materials and evaluation of their optical/electrical properties including photovoltaic performance requires a strong motivation and commitment.

Proposed structure of the general family of (III-V)(IV)3, compounds.

 

Al1-xBxPSi3 material system: The choice of Al1-xBxPSi3 is made on the basis of practical and theoretical considerations. First,the presence of boron in the AlPSi3 lattice is necessary to achieve perfect lattice matching to Si, which is an important requirement for photovoltaic applications. Second, boron may increase the band gap energy above the 1.3 eV due to the very large band gap of cubic BP. The Al1-xBxPSi3 alloys are synthesized epitaxially directly on Si(100) substrates by reactions of gaseous P(SiH3)3 and Al(BH4)3 using low pressure CVD. A model structure in (100) projection is shown at bottom right. The blue spheres denote the Si framework lattice. The BPSi3 and AlPSi3 building blocks are shown as red and green tetrahedra, respectively. The development of single phase crystalline and fully epitaxial crystals comprising light, earth abundant elements (Al,B,P and Si) have the potential for significant impact in the fields of refractory coatings and Si-based photovoltaics. The optical and electrical properties of these materials are currently being investigated.

Al1-xBxPSi3
The left panels shows that the reaction introduces isolated III-V units within the Si network. The right panel shows a model structure of Al1-xBxPSi3

The GaPSi3 system: Bottom left panel shows an absorption coefficient plot of the new material GaPSi3 illustrating direct edge above 1.6 eV. Insets show the crystal structure highlighting the GaPSi3 tetrahedra. We speculate that this finding is related to earlier predictions of strong direct absorption in Si/GaP superlattices. This research may open up an entirely new line of work in the long quest for direct-gap materials integrated with Si. Bottom right panels shows XRD plots containing the 004 peak of the GaPSi3 at slightly lower angles than Si (blue line) with thickness fringes on one side. The spectrum is compared with that of a commercial GaP sample (dashed line) indicating that the alloy lattice parameter is larger than Si and smaller than GaP as expected.

GaPSi3 system
Optical and structural properties of GaPSi3

The Ga(As1-xPx)Ge3 system: The choice of this system is partly motivated by the interest in exploring light emission with tunable wavelengths by incorporating P and As in the group IV sub-lattice. The synthesis was conducted by reactions of [D2GaN(CH3)2]2, with P(GeH3)3 and As(GeH3)3 yielding monocrystalline Ga(As1-xPx)Ge3 films with concentrations x = 0 1 thus providing samples for systematic band gap and lattice engineering over the entire composition range. Monocrystalline materials were integrated Si wafers using, for the first time, lattice-engineered Ge1-xSix templates which were specifically developed for this application. Ge1-xSix are the templates of choice for integration of defect-free Ga(As1-xPx)Ge3 epilayers because the lattice constant of Si (5.431 ) and Ge (5.657 ) closely match those of GaP (5.45 ), GaAs (5.654 ) and Ge (5.657 ) end members, allowing design of platforms with continuously tunable lattice parameters akin to those of the corresponding Ga(As1-xPx)Ge3 alloys. Figure below illustrates the Ge1-xSix on Si(100) buffer layer concept for epitaxy-driven synthesis of Ga(As1-xPx)Ge3 alloys..

Ga(As1-xPx)Ge3
 
 
Other Projects/Research Insterests:
 
Synthesis of main-group inorganic hydrides for applications in materials science.
 
 
 
 
Silicon-based photonic materials: growth and devices (photodetectors, modulators and emitters). 
 
 
 
 
Epitaxial integration of dissimilar materials with Si (including III-V and II-VI compounds for monolithic integration). 
 
 
 
 
Optoelectronic wide band gap semiconductors.
 
 
 
 
Solid-state inorganic chemistry based on light elements (refractory carbides, nitrides, borides and C-N frameworks).

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