Jianpeng Cheng

Growth of high quality and uniformity AlGaN/GaN heterostructures on Si substrates using a single AlGaN layer with low Al composition

By employing a single AlGaN layer with low Al composition, high quality and uniformity AlGaN/GaN heterostructures have been successfully grown on Si substrates by metal-organic chemical vapor deposition (MOCVD). The heterostructures exhibit a high electron mobility of 2150 cm2/Vs with an electron density of 9.3 × 1012 cm−2. The sheet resistance is 313 ± 4 Ω/◻ with ±1.3% variation. The high uniformity is attributed to the reduced wafer bow resulting from the balance of the compressive stress induced and consumed during the growth, and the thermal tensile stress induced during the cooling down process. By a combination of theoretical calculations and in situ wafer curvature measurements, we find that the compressive stress consumed by the dislocation relaxation (~1.2 GPa) is comparable to the value of the thermal tensile stress (~1.4 GPa) and we should pay more attention to it during growth of GaN on Si substrates. Our results demonstrate a promising approach to simplifying the growth processes of GaN-on-Si to reduce the wafer bow and lower the cost while maintaining high material quality.

High mobility AlGaN/GaN heterostructures grown on Si substrates using a large lattice-mismatch induced stress control technology

A large lattice-mismatch induced stress control technology with a low Al content AlGaN layer has been used to grow high quality GaN layers on 4-in. Si substrates. The use of this technology allows for high mobility AlGaN/GaN heterostructures with electron mobility of 2040 cm2/(V·s) at sheet charge density of 8.4 × 1012 cm−2. Strain relaxation and dislocation evolution mechanisms have been investigated. It is demonstrated that the large lattice mismatch between the low Al content AlGaN layer and AlN buffer layer could effectively promote the edge dislocation inclination with relatively large bend angles and therefore significantly reduce the dislocation density in the GaN epilayer. Our results show a great potential for fabrication of low-cost and high performance GaN-on-Si power devices.

Epitaxial growth of AlN films on sapphire via a multilayer structure adopting a low- and high-temperature alternation technique

Epitaxial growth of AlN films on c-sapphire using a multilayer structure has been investigated by metal–organic chemical vapor deposition adopting multiple alternation cycles of low- and high-temperature (LT–HT) growth. It is found that the surface morphology and crystal quality can be greatly improved using three alternation cycles with X-ray diffraction ω-scan full width at half maximum values of 311 and 548 arcsec for the (0002) and (10−12) peaks, respectively, which are induced by the alternation of the three-dimensional (3D) and two-dimensional (2D) growth modes caused by the LT–HT process. The first 3D–2D cycle is found to play a major role in threading dislocation reduction, while the second and third cycles mainly account for tensile stress relaxation.

Hysteresis phenomena of the two dimensional electron gas density in lattice-matched InAlN/GaN heterostructures

High-temperature transport properties in high-mobility lattice-matched InAlN/GaN heterostructures have been investigated. An interesting hysteresis phenomenon of the two dimensional electron gas (2DEG) density is observed in the temperature-dependent Hall measurements. After high-temperature thermal cycles treatment, the reduction of the 2DEG density is observed, which is more serious in thinner InAlN barrier samples. This reduction can then be recovered by light illumination. We attribute these behaviors to the shallow trap states with energy level above the Fermi level in the GaN buffer layer. The electrons in the 2DEG are thermal-excited when temperature is increased and then trapped by these shallow trap states in the buffer layer, resulting in the reduction and hysteresis phenomenon of their density. Three trap states are observed in the GaN buffer layer and CGa may be one of the candidates responsible for the observed behaviors. Our results provide an alternative approach to assess the quality of In...

Effects of light illumination on electron velocity of AlGaN/GaN heterostructures under high electric field

We have investigated the variation of electron velocity in AlGaN/GaN heterostructures depending on illuminating light intensity and wavelength. It is shown that the electron velocity at high electric field increases under above-band light illumination. This electron velocity enhancement is found to be related to the photo-generated cold holes which interact with hot electrons and thus accelerate the energy relaxation at high electric field. The results suggest an alternative way to improve the electron energy relaxation rate and hence the electron velocity in GaN based heterostructures.

Enhanced transport properties in InAlGaN/AlN/GaN heterostructures on Si (111) substrates: The role of interface quality

We have investigated the structural and transport properties of InAlGaN/AlN/GaN heterostructures grown on Si substrates. By depositing the AlN spacer layer at a low temperature after the growth interruption, the surface morphology and interface quality have been significantly improved. Electron mobilities of 1620 cm2/Vs at room temperature and 8260 cm2/Vs at 77 K are achieved while delivering a high electron sheet density of about 2.0 × 1013 cm−2, resulting in an extremely low sheet resistance of 186 Ω/□ at room temperature and 37 Ω/□ at 77 K. The experimental results evidence that it is the high interface quality that contributes to the improvement of electron transport properties. Our results provide an effective approach to obtain high quality InAlGaN/GaN heterostructures.

Spatial identification of traps in AlGaN/GaN heterostructures by the combination of lateral and vertical electrical stress measurements

We present a methodology and the corresponding experimental results to identify the exact location of the traps that induce hot electron trapping in AlGaN/GaN heterostructures grown on Si substrates. The methodology is based on a combination of lateral and vertical electrical stress measurements employing three ohmic terminals on the test sample structure with different GaN buffer designs. By monitoring the evolution of the lateral current during lateral as well as vertical stress application, we investigate the trapping/detrapping behaviors of the hot electrons and identify that the traps correlated with current degradation are in fact located in the GaN buffer layers. The trap activation energies (0.38–0.39 eV and 0.57–0.59 eV) extracted from either lateral or vertical stress measurements are in good agreement with each other, also confirming the identification. By further comparing the trapping behaviors in two samples with different growth conditions of an unintentionally doped GaN layer, we conclude ...

Vertical leakage induced current degradation and relevant traps with large lattice relaxation in AlGaN/GaN heterostructures on Si

We present a mechanism for the vertical leakage induced current degradation with identification of the properties of the relevant traps in AlGaN/GaN heterostructures on Si. The extent of the current degradation is determined by back-gating sweep measurements in double directions at different sweep rates and temperatures. It is found that the current degradation is only observed at relatively slow sweep rates and high temperatures. Time dependent back-gating measurements further suggest that the current degradation process is related to traps with long time constants. By comparing with the measurement results of samples on sapphire substrates, we confirm that the current degradation is caused by vertical leakage in heterostructures on Si. On the basis of the vertical leakage induced current degradation mechanism and in conjunction with the long-time degradation process, we measure both the trapping and detrapping processes of the relevant trap states to identify their properties. We find that there is a 0....

Anisotropic strain relaxation and high quality AlGaN/GaN heterostructures on Si (110) substrates

We have investigated the growth and relaxation mechanisms of anisotropic lattice misfit strain in AlN and GaN layers on Si (110) substrates. A qualitative model is proposed to explain the relaxation process. It is revealed that the anisotropic misfit strain is quickly relaxed in the low temperature AlN layer by the formation of interface misfit dislocations, small misoriented grains, and lattice distortion. As a result, isotropic properties and atomically smooth surface are observed in the high temperature AlN layer. Based on this isotropic AlN layer, a high quality GaN layer and AlGaN/GaN heterostructures with a high electron mobility of 2160 cm2/(V  · s) have been obtained. This work will have important impacts on the understanding of the epitaxy of isotropic semiconductor films on anisotropic substrates.

Hot electron induced non-saturation current behavior at high electric field in InAlN/GaN heterostructures with ultrathin barrier

The high-field transport characteristics of nearly lattice-matched InAlN/GaN heterostructures with different barrier thickness were investigated. It is found that the current in the InAlN/GaN heterostructures with ultrathin barrier shows unsaturated behaviors (or secondary rising) at high voltage, which is different from that of AlGaN/GaN heterostructures. This phenomenon is more obvious if the barrier thickness is thinner and the channel width is narrower. The experimental results demonstrate that it is the increasing carrier density excited from the more defect states by the hot electrons with larger electron saturation velocity that results in the unsaturated current behaviors in InAlN/GaN heterostructures. Our results pave a way for further optimizing InAlN barrier design and improving the reliability of InAlN/GaN HEMTs.