Strain–Stress Analysis of AlGaN/GaN Heterostructures With and Without an AlN Buffer and Interlayer

Abstract: The strain–stress analysis of Al_xGa_1?xN/GaN (x = 0.3) heterostructures with and without a high‐temperature HT‐AlN interlayer (IL) grown on sapphire (Al₂O₃) substrates and AlN buffer/Al₂O₃ templates via metal organic chemical vapour deposition (MOCVD) was carried out based on the precise measurement of the lattice parameters by using high‐resolution X‐ray diffraction (HRXRD). The a‐ and c‐lattice parameters were measured from the peak positions that were obtained by rocking the theta axis at the vicinity of the symmetric and asymmetric plane reflection angles, followed by the in‐plane and out‐of‐plane strains. Then, the biaxial and hydrostatic components were extracted from the total strain values that were obtained and were then discussed in the present study as the effect of the HT‐AlN buffer and IL. The AlN buffer layer (BL) affects the strain values of the AlGaN ternary layer (TL). A further effect was realized by inserting an AlN IL between GaN BL and AlGaN TL. However, the experimental results also show that an AlN IL changes the strain behaviour in the a‐ and c‐directions of the AlGaN TL from the tensile to compressive and the compressive to tensile type, respectively. These similar behaviours were observed in hydrostatic strain, biaxial strain and stress. Their reasons are explained with an effective a‐lattice parameter, post‐growth cooling and lattice and thermal mismatches.     

Low-Noise and High-Detectivity GaN UV Photodiodes With a Low-Temperature AlN Cap Layer

Here, we present the characteristics of a novel GaN- based ultraviolet (UV) photodiode (PD) with a low-temperature (LT) AIN cap layer. The dark leakage current for the PD with the LT-AIN cap layer was shown to be about four orders of magnitude smaller than that for the conventional PDs. It was found that we could achieve larger UV to visible rejection ratio by inserting an LT-AIN cap layer. It was also found that we could improve minimum noise equivalent power and maximum normalized detectivity of the PD by inserting an LT-AIN cap layer.     

AlGaN/GaNHEMT器件研究

叙述了ＡｌＧａＮ／ＧａＮ ＨＥＭＴ的特点及制造工艺,给出其测试结果,最大跨导￣１５７ｍｓ／ｍｍ,由Ｓ参数测量扒出器件的截止频率ｆｒ和最高振荡频率ｆｍａｘ分别为１２ＧＨｚ和２４ＧＨｚ。     

Undoped High-Resistance GaN Buffer Layer for AlGaN/GaN High-Electron-Mobility Transistors

Technical Physics Letters - It is shown that intentionally undoped high-resistance GaN buffer layers in AlGaN/GaN heterostructures with high electron mobility for transistors can be formed by...     

AlGaN/GaN HEMTs grown by ammonia MBE

The experimental technology and characteristics of domestic AlGaN/GaN high electron mobility transistors are described. The results show good prospects for further development.     

Multilayer AlN/AlGaN/GaN/AlGaN heterostructures for high-power field-effect transistors grown by ammonia MBE

The results of the optimization of the ammonia MBE technology of AlN/AlGaN/GaN/AlGaN heterostructures for high-power microwave field-effect transistors (FETs) are presented. The creation of technological systems of the EPN type for the deposition of group III nitrides by ammonia MBE, in combination with the development of optimum growth and postgrowth processes, make it possible to obtain AlN/AlGaN/GaN/AlGaN based heterostructures for high-power microwave FETs with the output static characteristics on the world best level. One of the main fields of application of the semiconductor heterostructures based on group III nitrides is the technology of high electron mobility transistors (HEMTs). Most investigations in this field have been devoted to the classical GaN/AlGaN structures with a single heterojunction. An alternative approach based on the use of double heterostructures with improved two-dimensional electron gas (2DEG) confinement offers a number of advantages, but such structures are usually characterized by a lower carrier mobility as compared to that in the single-junction structures. We succeeded in optimizing the double heterostructure parameters and growth conditions so as to obtain conducting channels with a 2DEG carrier mobility of 1450, 1350, and 1000 cm²/(V s) and a sheet electron density of 1.3 × 10¹³, 1.6 × 10¹³, and 2.0 × 10¹³ cm^?2, respectively. Experimental HEMTs with 1-μm-long gates based on the obtained multilayer heterostructure with a doped upper barrier layer exhibit stable current-voltage characteristics with maximum saturation current densities of about 1 A/mm and a transconductance of up to 180 mS/mm.     

AlGaN/GaN HEMTs passivated by Cat-CVD SiN Film

We demonstrate the excellent performance of a 140 W AlGaN/GaN HEMT in the C-band, which is passivated by a Cat-CVD SiN film. The interface trap density of the AlGaN surface passivated by Cat-CVD film after NH₃ treatment is 3 × 10¹² cm^− 2, which is the smallest of investigated deposition techniques. The lowest interface trap density achieved by the Cat-CVD technique makes it possible to operate the AlGaN/GaN HEMT in the C-band. We clarify that the Cat-CVD technique is necessary for developing future amplifiers.     

Reactive ion etching of GaN and AlGaN/GaN assisted by Cl2/BCl3

This work reports on the latest results of etching of different Al _x Ga_1?x N/GaN heterostructures in relation to percentage composition of aluminum. The etching processes were carried out in a reactive ion etching (RIE) system using the mixture of BCl₃/Cl₂/Ar. The topography of the heterostructures surfaces and the slope were controlled using atomic force microsopy (AFM) technique. The photoluminescence spectra were used to determine the surface damage and to calculate the Al content in AlGaN/GaN heterostructures commonly used for high electron mobility transistors (HEMTs) fabrication.     

Photoluminescence polarization in strained GaN/AlGaN core/shell nanowires

The optical polarization properties of GaN/AlGaN core/shell nanowire (NW) heterostructures have been investigated using polarization resolved micro-photoluminescence (μ-PL) and interpreted in terms of a strain-dependent 6 × 6 k·p theoretical model. The NW heterostructures were fabricated in two steps: the Si-doped n-type c-axis GaN NW cores were grown by molecular beam epitaxy (MBE) and then epitaxially overgrown using halide vapor phase epitaxy (HVPE) to form Mg-doped AlGaN shells. The emission of the uncoated strain-free GaN NW core is found to be polarized perpendicular to the c-axis, while the GaN core compressively strained by the AlGaN shell exhibits a polarization parallel to the NW c-axis. The luminescence of the AlGaN shell is weakly polarized perpendicular to the c-axis due to the tensile axial strain in the shell.     

Electrical and structural characterizations of non-polar AlGaN/GaN heterostructures

A non-polar AlGaN/GaN structure is a strong candidate for the high-voltage device that can operate in enhancement-mode compared to the depletion-mode operation that is practically unavoidable for a standard polar AlGaN/GaN structure. Growth of non-polar GaN is non-trivial and a two-step nucleation scheme was developed to produce high-quality non-polar a-plane AlGaN/GaN structures on r-plane sapphire. The anisotropic nature of non-polar GaN requires a modification to a typical polar GaN-based transistor fabrication process. A KOH wet etch proceeded by a dramatically different mechanism compared to the standard polar c-face AlGaN/GaN structure. This device with Pt/Au Schottky gate displayed a barrier height of 0.76 eV and an ideality factor of 4 at 20 °C.     

Analysis of the thermal behavior of AlGaN/GaN HEMTs

The thermal behavior of state-of-the-art multifinger AlGaN/GaN HEMTs grown on SiC is thoroughly analyzed under steady-state and dynamic conditions. Accurate 3-D FEM simulations – based on a novel in-house tool devised to automatically build the device mesh – are performed using a commercial software to explore the influence of various layout and technological solutions on the temperature field. An in-house routine is employed to determine the Foster/Cauer networks suited to describe the dynamic heat propagation through the device structure. To conclude, various experimental techniques are employed to assess the thermal resistance and to allow the monitoring of the thermal impedance versus time of the transistors under test.     

The improvement of GaN p-i-n UV sensor by 8-pair AlGaN/GaN superlattices structure

GaN with pairs of AlGaN/GaN superlattices (SLs) structure for p-i-n UV photo detector are fabricated on sapphire by metal organic chemical vapor deposition (MOCVD). For 8-pair AlGaN/GaN SLs not only eliminates cracking through this strain management, but it also significantly decreases the threading dislocation density by acting itself as an effective dislocation filter. The related structure has exhibited excellent film qualities such as enhanced crystallinity, lower specific contact resistance, lower etching pit density or mean roughness in the film. GaN p-i-n diode fabricated with 8-pair SLs, the dark current of device is reduced by two orders of magnitude than that without SLs structure at reverse bias of −3 V. Moreover, the peak UV responsivity is 0.12 A/W, which is higher than that without SLs is 0.07 A/W at 360 nm. The rejecting ratio is also by two orders of magnitude higher than that without SLs structure.     

III-Nitrides growth and AlGaN/GaN heterostructures on ferroelectric materials

The growth of III-nitrides on the ferroelectric materials lithium niobate (LN) and lithium tantalate (LT) via molecular beam epitaxy (MBE) using rf plasma source has been investigated. We have found that gallium nitride (GaN) epitaxial layers have a crystalline relationship with lithium niobate (tantalate) as follows: (0 0 0 1) GaN || (0 0 0 1) LN (LT) with [10−10] GaN || [11−20] LN (LT). The surface stability of LN and LT substrates has been monitored by in situ spectroscopic ellipsometry in the vacuum chamber. Three different temperature zones have been discerned; surface degas and loss of OH group (100–350 °C); surface segregation/accumulation of Li and O-species (400–700 °C); surface evaporation of O-species and Li desorption (over 750 °C). However, LT shows only surface degassing in the range of 100–800 °C. Therefore, congruent LN substrates were chemically unstable at the growth temperature of 550–650 °C, and therefore developed an additional phase of Li-deficient lithium niobate (LiNb₃O₈) along with lithium niobate (LiNbO₃), confirmed by X-ray diffraction. On the other hand, LT showed better chemical stability at these temperatures, with no additional phase development. The structural quality of GaN epitaxial layers has shown slight improvement on LT substrates over LN substrates, according to X-ray diffraction. Herein, we demonstrate AlGaN/GaN heterostructure devices on ferroelectric materials that will allow future development of multifunctional electrical and optical applications.     

AlGaN／GaN异质结2DEG载流子输运

本言语采用雷－丁平衡方程理论,考虑杂质散射、声学波形变势散射、声学波压电散射、极化光学波散射等散射机制,计算了ＡｌＧａＮ／ＧａＮ异质结二维电子气（２ＤＥＧ）在０－３００Ｋ温度范围内的低场迁率以及电子漂移速度和电子温度随外加电场的变化关系,同时本文计算了有隔离层的调制掺杂异质结构（ＭＤＨ）中低温下（Ｔ＝４Ｋ）低场迁移率随隔离层厚度的变化关系。计算结果表明在低温下ＡｌＧａＮ／ＧａＮ异质结构的低场迁移率     

Buffer-trapping effects on current slump in AlGaN/GaN HEMTs

Two-dimensional transient analyses of GaN MESFETs and AlGaN/GaN HEMTs are performed in which a deep donor and a deep acceptor are considered in a semi-insulating buffer layer. Calculated transient characteristics are compared between the two FETs, and it is shown that the deep levels affect the results essentially in a similar way. Quasi-pulsed I–V curves are derived from the transient characteristics, and are compared with steady-state I–V curves. It is shown that so-called current slump is more pronounced when the deep-acceptor density in the buffer layer is higher and when an off-state drain voltage is higher, because trapping effects become more significant. It is suggested that to minimize current slump in GaN-based FETs, an acceptor density in a semi-insulating GaN layer should be made low.     

Refractive index changes in AlGaN/GaN heterostructure field-effect transistors

Using the envelope wavefunction approximation and the compact density matrix formalism, we have investigated theoretically the linear and nonlinear refractive index changes in AlGaN/GaN quantum well heterostructures aimed for designing electro-optical modulators. The confining potential in the heterostructures is assumed to be semiparabolic. Simulated results reveal that the refractive index changes strongly depend on both the Al composition and the delta-doping concentration. On the other hand an applied electric field further enhances the refractive index changes. Compared with AlGaAs/GaAs heterostructures and quantum dots, the amount of the refractive index is larger in the AlGaN/GaN quantum well heterostructures studied. The fact to have a large refractive index change leads to the use of relatively weaker incident beam intensities.     

Structural characterization of AlGaN/GaN superlattices by three-beam X-ray diffraction

Three-beam X-ray diffraction in AlGaN/GaN superlattices (SLs) grown by metalorganic chemical vapor deposition on c-sapphire has been measured in the Renninger scheme. The primary beam corresponds to a forbidden 0001 reflection. Then, θ-scan curves were measured at the maximum of each three-beam diffraction peak. The average parameters a and c of SLs have been determined using the angular positions of three-beam diffraction peaks on the Renninger diagram (φ-scan curves). It is shown that a diffraction pattern with satellites on the θ-2θ curve of 0001 reflection can be obtained in the azimuthal position of three-beam diffraction. The angular widths of three-beam diffraction peaks measured in both φ and θ scans have been analyzed as related to the defect structure of layers. On this basis, a new method of determining the structural parameters of SLs is proposed.     

AlGaN/GaN肖特基二极管中金属电极的制备方法研究

本文主要研究AlGaN/GaN二极管制备工艺中Ohmic Contact金属和Schottky Contact金属电极的制备,主要讨论了金属电极材料的选取,退火温度和退火时间对接触电阻的影响,最后得出了Ohmic Contact的比接触电阻和TiN的淀积参数。     

Suitability of thin-GaN for AlGaN/GaN HEMT material and device

Journal of Materials Science - In this study, we report about the suitability of thin-GaN (~ 200 nm) for AlGaN/GaN HEMT (High Electron Mobility Transistor) material and device....     

Nb-Ti/Al/Ni/Au based ohmic contacts to AlGaN/GaN

In this paper, we report on a novel Nb-Ti/Al/Ni/Au metallic system proposed to form ohmic contact to AlGaN/GaN heterostructure. The metallic system uses deposition of thin niobium layer as the first layer in contact with the AlGaN barrier layer before deposition of the conventional Ti/Al/Ni/Au metallic system. The fabrication and electrical characterization of the Nb-Ti/Al/Ni/Au based ohmic contacts are presented. We have shown that Nb-based ohmic contacts at optimal alloying temperatures seem to be superior to that of conventional Ti/Al/Ni/Au in both surface morphology and contact resistivity evaluation. Auger Electron Spectroscopy (AES) and Secondary Ion Mass Spectroscopy (SIMS) are also used to evaluate the improved ohmic contact formation.