Bill Nemeth

GaN epitaxy on thermally treated c-plane bulk ZnO substrates with O and Zn faces

ZnO is considered as a promising substrate for GaN epitaxy because of stacking match and close lattice match to GaN. Traditionally, however, it suffered from poor surface preparation which hampered epitaxial growth in general and GaN in particular. In this work, ZnO substrates with atomically flat and terrace-like features were attained by annealing at high temperature in air. GaN epitaxial layers on such thermally treated basal plane ZnO with Zn and O polarity have been grown by molecular beam epitaxy, and two-dimensional growth mode was achieved as indicated by reflection high-energy electron diffraction. We observed well-resolved ZnO and GaN peaks in the high-resolution x-ray diffraction scans, with no Ga2ZnO4 phase detectable. Low-temperature photoluminescence results indicate that high-quality GaN can be achieved on both O- and Zn-face ZnO.

Comparative study of the (0001) and (0001) surfaces of ZnO

The authors compare the surface and optical properties of the Zn-polar (0001) and O-polar (0001¯) surfaces of bulk ZnO samples. For optical characterization, steady-state photoluminescence using a He–Cd laser was measured at 15 and 300K. At room temperature, the (0001¯) surface demonstrates nearly double the near-band-edge emission intensity seen for the (0001) surface. Using scanning Kelvin probe microscopy, the authors have measured surface contact potentials of 0.39±0.05 and 0.50±0.05V for the (0001) and (0001¯) surfaces, respectively. The resulting small difference in band bending for these two surfaces indicates that charge transfer between the surfaces is not a dominant stabilizing mechanism. Conductive atomic force microscopy studies show enhanced reverse-bias conduction in localized regions on the (0001¯) vs (0001) surface. The differences in surface conduction and band bending between the two polar surfaces can be attributed to their chemical interactions with hydrogen and water in the ambient.

III-nitrides on oxygen- and zinc-face ZnO substrates

The characteristics of III-nitrides grown on zinc- and oxygen-face ZnO by plasma-assisted molecular beam epitaxy were investigated. The reflection high-energy electron diffraction pattern indicates formation of a cubic phase at the interface between III-nitride and both Zn- and O-face ZnO. The polarity indicates that Zn-face ZnO leads to a single polarity, while O-face ZnO forms mixed polarity of III-nitrides. Furthermore, by using a vicinal ZnO substrate, the terrace-step growth of GaN was realized with a reduction by two orders of magnitude in the dislocation-related etch pit density to ∼108cm−2, while a dislocation density of ∼1010cm−2 was obtained on the on-axis ZnO substrates.

Effect of thermal treatment on ZnO substrate for epitaxial growth

ZnO is a highly efficient photon emitter, and has optical and piezoelectric properties that are attractive for a variety of applications in sensors and potentially optoelectronic devices such as emitters. Due to its identical stacking order and close lattice match to GaN, it is also being developed as a substrate material for GaN epitaxy. However, the surface finish of the ZnO is such that much of the damage induced by sawing and follow up mechanical polishing remains. We developed a thermal treatment method to eliminate surface damage on the 0 face of ZnO (0 0 0 1) to prepare it for epitaxial growth. Atomic force microscopy images of ZnO (0 0 0 1) annealed at 1050 °C for 3 h etc. show that residual scratches from mechanical polishing are removed and atomically flat, terrace-like surfaces are attained. In addition, low-temperature photoluminescence and high-resolution X-ray diffraction measurements have been employed to investigate the effect of annealing on ZnO substrates.

Growth of GaN on ZnO for Solid State Lighting Applications

In this work, ZnO has been investigated as a substrate technology for GaN-based devices due to its close lattice match, stacking order match, and similar thermal expansion coefficient. Since MOCVD is the dominant growth technology for GaN-based materials and devices, there is a need to more fully explore this technique for ZnO substrates. Our aim is to grow low defect density GaN for efficient phosphor free white emitters. However, there are a number of issues that need to be addressed for the MOCVD growth of GaN on ZnO. The thermal stability of the ZnO substrate, out-diffusion of Zn from the ZnO into the GaN, and H2 back etching into the substrate can cause growth of poor quality GaN. Cracks and pinholes were seen in the epilayers, leading to the epi-layer peeling off in some instances. These issues were addressed by the use of H2 free growth and multiple buffer layers to remove the cracking and reduce the pinholes allowing for a high quality GaN growth on ZnO substrate.

Low temperature Si/SiO x /pc-Si passivated contacts to n-type Si solar cells

We describe the design, fabrication, and results of low-recombination, passivated contacts to n-type silicon utilizing thin SiO_x, and plasma enhanced chemical vapor deposited doped polycrystalline-silicon (pc-Si) layers. A low-temperature silicon dioxide layer is grown on both surfaces of an n-type CZ wafer to a thickness of <;20 Å. Next, a thin layer of P-doped plasma enhanced chemical vapor deposited amorphous silicon (n/a-Si:H) is deposited on top of the SiO_x. The layers are annealed to crystallize the a-Si:H and diffuse H to the Si/SiO_x interface, after which a metal contacting layer is deposited over the conducting pc-Si layer. The contacts are characterized by measuring the recombination current parameter of the full-area contact (J_o,contact) to quantify the passivation quality, and the specific contact resistivity (ρ_contact). The Si/SiO_x/pc-Si contact has an excellent J_o,contact = 30 fA/cm² and a good ρ_contact = 29.5 mOhm-cm². Separate processing conditions lowered J_o,contact to 12 fA/cm². However, the final metallization can substantially degrade this contact and has to be carefully engineered. This contact could be easily incorporated into modern, high-efficiency solar cell designs, benefiting performance and yet simplifying processing by lowering the temperature and growth on only one side of the wafer.

High quantum efficiency of photoluminescence in GaN and ZnO

The quantum efficiency (QE) of photoluminescence (PL) has been estimated in GaN and ZnO samples. A Si-doped GaN layer grown by molecular beam epitaxy (MBE) exhibited the highest QE of about 90% at low temperatures. Recombination via the shallow donor-acceptor pair transitions dominated in this sample. In contrast, a bulk ZnO crystal with the QE of PL of about 85% contained almost no defect- or impurity-related PL signatures besides the emission attributed to free and bound excitons. The sources of radiative and nonradiative recombination in GaN and ZnO are discussed.

Some challenges in making accurate and reproducible measurements of minority carrier lifetime in high-quality Si wafers

Measurement of the minority carrier lifetime (τ) of high-quality wafers (having bulk minority carrier lifetime, τb > few milliseconds) requires surface passivation with very low surface recombination velocity, typically <; 1cm/s. Furthermore, for mapping large (e.g., 156 x156 mm) wafers, the passivation must also be stable and uniform over the entire wafer surfaces. These are very demanding requirements and it is a common experience that they are very difficult to achieve. Yet, they are necessary for performing defect analyses of the current N-type wafers. To understand the problems associated with these measurements, we have studied effect of wafer preparation (cleaning procedures, handling) and the passivation characteristics (stability, sensitivity to light, thickness of the passivation medium required for stable passivation) for many commonly used passivation media-iodine-ethanol (IE), quinhydrone-methanol (QHM), aluminum oxide (Al2O3), amorphous-silicon (a-Si), and silicon dioxide (SiO2). Here, we will discuss main factors that influence the accuracy and repeatability of lifetime measurements.

Bulk defect generation during B-diffusion and oxidation of CZ wafers: Mechanism for degrading solar cell performance

We describe results of our experimental study to investigate the effect of B diffusion and drive-in/oxidation on minority carrier lifetime of the wafer. We have observed that B diffusion generates stacking faults that can be attributed to injection of Si interstitials into the wafer by formation of a boron rich layer at the wafer surface. These Si interstitials are also believed to enhance interactions between the native point defects and impurities (such as O, Fe) in the wafers during subsequent processing leading to the development of swirl patterns. Spatial variation of the lifetime degradation follows the point defect interactions and impurity segregation/precipitation. Lifetime can be partially recovered by Phosphorous (P) gettering. The overall effect on the cell performance due to Si interstitial generation, impurity/point defect interactions, and P-gettering is briefly discussed.

Dielectric stack passivation on boron- and phosphorus-diffused surfaces and 20% efficient PERT cell on n-CZ silicon substrate

We present a surface passivation study of a B-diffused emitter and P-diffused back-surface field (BSF) of n-CZ Si substrates. The optimized passivation layers are subsequently incorporated into a 20%-efficient passivated emitter, rear totally-diffused (PERT) cell with V_oc of 672 mV. On the P-diffused, concentrated KOH-planarized BSF side, we compare different passivating plasma enhanced chemical vapor deposition (PECVD) SiN_x layer compositions. We demonstrate that a favorable combination of best passivation quality is achieved by a stack of thermal oxide grown at ~700°C, followed by a bilayer SiN_x, consisting of stoichiometric PECVD nitride and capped with Si-rich nitride, or H-dilution nitride. The stack results in surface passivation quality of J_o ~ 17 fA/cm² for bilayer SiN_x and 14 fA/cm² for H-SiN_x on lightly P-doped BSF, and is very resistive to HF-containing wet etches. Surface preparation, deposition parameters, and post-growth annealing collaborate to define the effectiveness of the passivation. Their optimization is critical for integration of SiN_x:H into our high-efficiency solar cells. On the B-diffused textured emitter side, we use atomic layer deposition (ALD)-deposited Al₂O₃ for surface passivation and low-temperature stoichiometric PECVD SiN_x for the anti-reflection coating. We discuss deposition conditions and thermal treatments for both ALD Al₂O₃ and PECVD nitride that result in the optimized passivation resulting in J_o ~ 52 fA/cm² and that prevent the blistering of the film.