Source resistance reduction of AlGaN-GaN HFETs with novel superlattice cap layer

We have developed a novel AlGaN-GaN heterojunction field effect transistor (HFET) with an ultralow source resistance by employing the novel superlattice (SL) cap structure. The particular advantage of the SL cap, i.e., the existence of multiple layers of the polarization-induced two-dimensional electron gas (2DEG) with high mobility and high concentration at each AlGaN-GaN interface, is fully exploited for lowering the lateral resistance and the potential barrier at the interface of the SL cap and the HFET barrier layer. By designing the AlGaN-GaN thickness ratio, we have established a method to obtain the optimized SL structure and have achieved an extremely low source resistance of 0.4 /spl Omega//spl middot/mm which is lower not only than HFETs with the conventional structure but also than those with the n-GaN cap structure. The SL cap HFET fabricated on a sapphire substrate exhibited excellent dc and RF performance, i.e., maximum transconductance of over 400 mS/mm, maximum drain current of 1.2 A/mm, a cutoff frequency of 60 GHz, a maximum frequency of oscillation of 140 GHz, and a very low noise figure minimum of 0.7 dB at 12 GHz.     

Polyimide passivated AlGaN-GaN HFETs with 7.65 W/mm at 18 GHz

Current metal-organic chemical vapor deposition-grown AlGaN-GaN heterojunction field-effect transistor devices suffer from threading dislocations and surface states that form traps, degrading RF performance. A passivation scheme utilizing a polyimide film as the passivating layer was developed to reduce the number of surface states and minimize RF dispersion. Continuous-wave power measurements were taken at 18 GHz on two-finger 0.23-/spl mu/m devices with 2/spl times/75 /spl mu/m total gate width before and after passivation yielding an increase from 2.14 W/mm to 4.02 W/mm in power density, and 12.5% to 24.47% in power added efficiency. Additionally, a 2/spl times/25 /spl mu/m device yielded a peak power density of 7.65 W/mm at 18 GHz. This data suggests that polyimide can be an effective passivation film for reducing surface states.     

The improvement of DC performance in AlGaN-GaN HFETs with isoelectronic Al-doped channels

Enhancement of electrical properties of an AlGaN-GaN heterostructure was achieved through isoelectronic Al-doping of the undoped channel layer during the growth by metal-organic chemical vapor deposition. The two-dimensional electron gas mobility was increased from 3390 to 4870 cm/sup 2//V/spl middot/s at 77 K, and the crystal quality was significantly improved as Al atoms were incorporated in the undoped GaN film. The AlGaN-GaN HFETs were fabricated on this material structure and exhibited a maximum drain current of 909 mA/mm, and a maximum transconductance of 232 mS/mm, corresponding to an increase of 30% and 21%, respectively.     

Low-frequency 1/f noise and persistent transients in AlGaN-GaN HFETs

Persistent photoresponse transients and low-frequency 1/f noise were measured in barrier-controlled devices such as AlGaN-GaN heterostructure field-effect transistors (HFETs). The persistent transients and 1/f noise were observed in drain and gate currents. A model describing trapping in a barrier-controlled device introduced, and the appropriate transient evolution and the noise spectra developed. Excellent agreement was obtained between the experimental measurements and the predicted temporal response and noise spectra. Finally, the correlation between drain and gate 1/f noise was measured, confirming that the same noise source (fluctuation in surface potential) is responsible to both currents.     

Thermal management of AlGaN-GaN HFETs on sapphire using flip-chip bonding with epoxy underfill

Self-heating imposes the major limitation on the output power of GaN-based HFETs on sapphire or SiC. SiC substrates allow for a simple device thermal management scheme; however, they are about a factor 20-100 higher in cost than sapphire. Sapphire substrates of diameters exceeding 4 in are easily available but the heat removal through the substrate is inefficient due to its low thermal conductivity. The authors demonstrate that the thermal impedance of GaN based HFETs over sapphire substrates can be significantly reduced by implementing flip-chip bonding with thermal conductive epoxy underfill. They also show that in sapphire-based flip-chip mounted devices the heat spread from the active region under the gate along the GaN buffer and the substrate is the key contributor to the overall thermal impedance.     

Thermal modeling and measurement of AlGaN-GaN HFETs built on sapphire and SiC substrates

We present thermal modeling and measurement results of AlGaN-GaN heterojunction field effect transistors fabricated on sapphire and SiC substrates, respectively. The device structures are identical except for the substrate material used to grow the AlGaN-GaN heterostructure. One objective is to study the effect of substrate material on the thermal and electrical performance of the resulting devices. To compute the temperature profiles, in-house PAMICE code developed for a three-dimensional structure was used. To measure the temperatures on the chip surface, nematic liquid crystal thermography was used. This technique is nondestructive and can be performed in realtime during device operation. It has submicrometer spatial resolution and /spl plusmn/1/spl deg/C temperature accuracy. The measured temperatures agree well with the calculated ones. The relationship between the measured temperature and power is almost linear for both types of devices. The junction-to-case thermal resistance of the device fabricated on sapphire substrate is 4.4 times that of the device built on SiC substrate.     

高Al组分AlGaN材料优化生长与组分研究

研究了金属有机化合物化学气相沉积(MOCVD)系统外延高Al组分较厚AlGaN薄膜材料的生长技术。实验发现,AlGaN/GaN结构中的AlGaN材料的相分离现象可能是由于过低的生长V/III以及材料所受的张应力状态所致,而V/III过高时则会出现Al源的并入效率饱和。采用AlN过渡层技术,外延生长了表面无裂纹的45%Al组分较厚(100～200nm)AlGaN薄膜材料。所得材料的Al组分与气相Al组分相同,(0002)面X射线衍射(XRD)双晶摇摆曲线半高宽(FWHM)为376arcsec,并发现AlN过渡层的质量影响着其上AlGaN材料的Al组分与晶体质量。实验观察到AlGaN材料的表面形貌随着样品中Al组分的增加从微坑主导模式逐步转变为微裂主导模式,采用AlN过渡层可延缓这一转变。     

Chromium thin film as a barrier to the interaction of Pd2Si with Al

Backscattering spectrometry with 2.3 and 2.0 MeV ⁴He⁺ have been used to study the role of Cr as a barrier in the interaction of Pd₂Si with Al. Samples of palladium silicide (Pd₂Si) grown on Si 〈100〉 single crystal and Al evaporated on top, in that order, showed a substantial intermixture of Pd₂Si and Al, and a non-uniform erosion of the Pd₂SiSi interface when heated between 300 and 450°C. With a thin layer of Cr(300–1500 Å) interposed between Pd and Al intermixing of Pd₂Si and Al was suppressed for temperatures up to 500°C and times up to 2 hr. In these samples distinct sublayers of Pd₂Si, CrSi₂ and CrAl_x (where the values of x depends on the relative thicknesses of Al and Cr) are formed. We have noted that whenever there is a thin unreached Cr layer the spectra of the distinct sublayers show sharp boundaries.     

Increase in barrier height of Al/n-GaAs contacts induced by high current

High forward-gate current (HFGC) density induces an increase of the Al/n-GaAs barrier height from 0.8 to 0.96 eV, thus suggesting the formation of an GaxAl1-xAs layer at the interface. Results show that this interaction is more enhanced by the electron current from the semiconductor to the metal than by thermal treatments. The intense electron flow is believed to contribute to the breaking of the interfacial oxide layer present at the metal-semiconductor interface, thus promoting Al/GaAs interdiffusion. Data were obtained on power MESFET's with Al metallized gate.     

Variations of temperature coefficient and noise in thin Al and Al/Si resistors subjected to high current density

Electromigration (EM) in thin Al and Al/Si resistors was investigated by measuring the effect of the stress on the resistance, the temperature coefficientα, the low-frequency noise spectrum (20 mHz-1 Hz) and, for some Al samples, on the residual resistance atT = 11 K. It was found that the temperature coefficient variations versus the stress time have a different behavior for Al and Al/Si, that is an oscillation in the first part of the life of the sample and decreases in the last one, in the first case, whereas, in the second, only the oscillation was detected. This behavior, observed for the first time, has been modeled by means of the Mayadas theory, by supposing a Gaussian distribution of the reflection coefficient for electrons at the grain boundaries. The measurement of the residual resistance also supports the hypothesis that the behavior ofa andR _o (the resistance value at 273 K) is mainly due to the modification of the grain-boundary structure and cannot be explained by taking in account only the variation of the residual resistivity. Besides, the noise measurements give a further proof that the origin of the 1/fg spectra in thin films subjected to high current density must be ascribed to phenomena occurring at the grain-boundaries which are correlated to the phenomena causing temperature coefficient variations. This work was supported by the Progetto Finalizzato MADESS, CNR, Italy.     

The effects of isoelectronic Al doping and process optimization for the fabrication of high-power AlGaN-GaN HEMTs

In order to improve the electrical characteristics of AlGaN-GaN heterostructures for applications in high electron mobility transistors (HEMTs), high-quality AlGaN-GaN was grown by way of metal-organic chemical vapor deposition on sapphire. We applied isoelectronic Al doping into the GaN-channel layers of modified AlGaN-Al-doped GaN channel-GaN heterostructures. We then compared the electrical performance of the fabricated heterostructures with those of conventional AlGaN-GaN heterostructures. The AlGaN-GaN HEMTs that were fabricated achieved power densities of up to 4.2 W/mm, some of the highest values ever reported for 0.25-/spl mu/m gate length AlGaN-GaN HEMTs. These devices exhibited a maximum drain current density of 1370 mA/mm, a high transconductance of 230 mS/mm, a short-circuit current gain cutoff frequency (f/sub T/) of 67 GHz, and a maximum frequency of oscillation (f/sub max/) of 102 GHz.     

高Al组分In1-xAlx Sb/InSb的MBE生长研究

高Al(10%~15%)组分的In1-xAlx Sb层可作为势垒层以抑制隧穿暗电流、提高器件工作温度而显得很重要。采用分子束外延的方法对InSb(100)衬底高Al组分的In1-xAlx Sb/InSb外延生长进行了实验探索,确定出了Al组分(约12.5%)并讨论了Al组分梯度递变的In1-xAlx Sb缓冲层和生长温度对外延薄膜质量的影响。     

YSZ/Al_2O_3复合薄膜高温绝缘层的研究

采用电子束蒸发、射频磁控溅射、等离子喷涂等方法,在镍基高温合金基底上制备YSZ(质量分数12%Y2O3稳定的Zr O2)、Al2O3复合薄膜结构绝缘层,并研究了复合薄膜结构绝缘层在室温到800℃范围内的绝缘特性,以及高温对复合薄膜晶体结构和表面形貌的影响。结果表明:晶态YSZ/非晶态YSZ/Al2O3结构绝缘层在室温下的绝缘电阻大于1.2 GΩ,在800℃大气环境下有150 kΩ左右的绝缘电阻。在室温到800℃范围内,随温度升高其绝缘电阻呈近指数下降的变化规律。经过在800℃大气环境中热处理8 h,YSZ的立方相结构未发生改变,Al2O3表面十分致密,表明该复合结构绝缘层薄膜具有良好的高温绝缘性能和稳定性。     

Small-scale plasticity in thin Cu and Al films

Flow stresses in thin metal films significantly exceed the flow stresses of their bulk counterparts. In order to identify the underlying deformation mechanisms and correlate them with microstructure, we analysed epitaxial and polycrystalline Cu and Al thin films. The films (100–2000 nm thickness) were magnetron sputtered on (0001) -Al₂O₃ single crystals or on nitrided and oxidised (001) Si substrates. For epitaxial films, the flow stress measurements, which were obtained from substrate-curvature tests, agree with predictions from a dislocation-based model [1 and 2], whereas for polycrystalline films the stresses measured for film thicknesses down to 400 nm are much higher than predicted. However, thinner films reveal a plateau in room temperature flow stress. This behavior, as well as the stress–temperature evolution of the various films will be discussed in terms of existing theories for plasticity in thin metal films, and under consideration of recent in situ transmission electron microscopy studies.     

InAs-based heterostructure barrier varactor diodes with In0.3Al0.7As0.4Sb0.6 as the barrier material

InAs-based heterostructure barrier varactor (HBV) diodes with In_0.3Al_0.7As_0.4Sb_0.6 as the barrier material are demonstrated. Current–voltage and capacitance–voltage characteristics, as well as S-parameters, of HBV diodes with varying barrier thicknesses are examined. Maximum capacitance values and maximum-to-minimum capacitance ratios greater than those predicted by traditional HBV models were measured. The HBVs’ unconventional behavior in terms of charge accumulation layers adjacent to the wide bandgap barrier is discussed.     

The In/PbTe barrier structures with a thin intermediate insulating layer

Epitaxial n-PbTe layers were grown on BaF₂ {111} single-crystal substrates by hot-wall epitaxy from the gaseous phase. These layers were kept in atmospheric air for 15–30 days, after which In and protective BaF₂ layers were deposited. Current-voltage characteristics and photoelectric sensitivity spectra of the In/n-PbTe barrier structures were measured in the temperature range T=80–300 K. Based on the experimental results, a model of charge transport is suggested and the effective barrier height ? _b ^eff , the insulator layer thickness δ, and the surface-state density D _S are determined.     

Cu contact on NiSi/Si with thin Ru/TaN barrier

Thin Ru(5 nm)/TaN(15 nm) bi-layer was sputtered on the NiSi/Si substrate as a diffusion barrier in the copper contact structure. The barrier properties were investigated through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) and electrical measurement. The whole Cu/Ru/TaN/NiSi/Si structure has a good thermal stability until after annealing at 450 °C. The Schottky barrier measurement shows that the leakage current increases after 450 °C annealing and after 500 °C annealing the barrier fails. Failure mechanism of the barrier stack is discussed.     

高Al含量AlGaN多层外延材料的应变与位错密度研究

文中利用X射线三轴衍射测试手段对高Al ( x≥0. 45)含量p2i2n结构的Alx Ga1 - xN外延材料进行测试,并结合倒易空间图(RSM)和PV函数法对Alx Ga1 - xN外延材料进行评价。首先通过对RSM的定性分析,给出多层外延材料中不同Al组分Alx Ga1 - xN材料的应变状态和位错密度等信息,然后结合PV函数法拟合了从RSM中分离出的摇摆曲线,通过拟合过程准确地计算了多层结构中不同组分的Alx Ga1 - xN材料的纵向和横向应变量与螺位错密度,测试及计算结果都表明:多层p2i2n结构的Alx Ga1 - xN外延材料的应变与位错密度与单层结构相差较大,表明层与层之间的应变和位错相互作用对各层的应变的位错密度有重要的影响。     

High-transconductance p-channel AlGaAs/GaAs HFETs with low-energyberyllium and fluorine co-implantation self-alignment

The fabrication and electrical characteristics of p-channel AlGaAs/GaAs heterostructure FETs with self-aligned p⁺ source-drain regions formed by low-energy co-implantation of Be and F are reported. The devices utilize a sidewall-assisted refractory gate process and are fabricated on an undoped AlGaAs/GaAs heterostructure grown by MOVPE. Compared with Be implantation alone, the co-implantation of F⁺ at 8 keV with 2×10¹⁴ ions/cm² results in a 3× increase in the post-anneal Be concentration near the surface for a Be⁺ implantation at 15 keV with 4×10¹⁴ ions/cm². Co-implantation permits a low source resistance to be obtained with shallow p⁺ source-drain regions. Although short-channel effects must be further reduced at small gate lengths, the electrical characteristics are otherwise excellent and show a 77-K transconductance as high as 207 mS/mm for a 0.5-μm gate length     

Nitride-based green light-emitting diodes with high temperature GaN barrier layers

High-quality InGaN-GaN multiquantum well (MQW) light-emitting diode (LED) structures were prepared by temperature ramping method during metalorganic chemical vapor deposition (MOCVD) growth. It was found that we could reduce the 20-mA forward voltage and increase the output intensity of the nitride-based green LEDs by increasing the growth temperature of GaN barrier layers from 700/spl deg/C to 950/spl deg/C. The 20-mA output power and maximum output power of the nitride-based green LEDs with high temperature GaN barrier layers was found to be 2.2 and 8.9 mW, respectively, which were more than 65% larger than those observed from conventional InGaN-GaN green LEDs. Such an observation could be attributed to the improved crystal quality of GaN barrier layers. The reliability of these LEDs was also found to be reasonably good.