X波段AlGaN/GaN HEMT器件物理与相关实验研究
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摘要
具有高温、高频和大输出功率能力的宽禁带AlGaN/GaN HEMT器件已成为国内外研究的热点课题。由于异质外延生长的AlGaN/GaN HEMT材料存在晶格和热失配问题,特别是GaN基异质结器件存在很强的自发极化和压电极化效应,从而导致器件中存在许多物理效应的作用机制仍不明确,而相关的实验如材料表征、应变分析、离子注入掺杂、欧姆接触和电流崩塌等仍然存在许多科学问题需要进一步深入研究。
本论文围绕X波段AlGaN/GaN HEMT所涉及的器件物理和相关实验等科学技术问题开展了基础研究,取得的主要研究结论如下。
1、基于对AlGaN/GaN HEMT自发极化和压电极化效应的研究,导出了异质结中的极化强度和晶格常数及弹性系数之间的关系;利用电中性平衡原理,建立了非故意掺杂AlGaN/GaN HEMT器件结构中极化面电荷密度和2DEG之间的物理模型。
2、应用高分辨X射线衍射技术(HRXRD)的GaN材料晶格参数精确测量方法,深入研究了GaN基异质结构材料水平和垂直方向的应变情况;同时利用Williamson-Hall方法测量了外延材料的镶嵌结构参数,明确了材料中存在的位错类型,对螺位错和刃位错的位错密度进行了精确计算,获得总的位错密度为10~9cm~(-2)量级。
3、通过Si离子注入GaN材料的掺杂实验,在较低温度下(<1100°C)利用快速热退火技术实现了GaN的重掺杂目的,为制作高质量欧姆接触提供了新的技术途径。研究表明,当注入能量100keV和注入剂量10~(16)cm~(-2)的样品在1100°C快速热退火处理后可实现重掺杂,其载流子面密度为2×10~(15)cm~(-2)、方块电阻100Ω/□,而穿透位错密度降低至1.55×10~9cm~(-2)。同时,利用光致发光PL谱测试发现了能量分别为2.61eV与2.67eV蓝光发光(BL)谱线,这与前人研究的结果有所不同。进一步研究表明,2.61eV蓝光BL发光是由施主能级ON到深受主复合能级VGa-SiGa的跃迁发光产生的,而2.67eV BL带发光是由20meV左右的SiGa浅施主能级向VGa-ON复合深受主能级之间电子跃迁辐射发光所产生。
4、在非故意掺杂GaN上实现了低比接触电阻率的高质量多层金属欧姆接触。采用Ti(15nm)/Al(220nm)/Ni(40nm)/Au(50nm)四层金属在非故意掺杂GaN上进行欧姆接触实验研究,在N2气氛中经温度900°C快速热退火1min后获得了最低的比接触电阻率为1.26×10-7Ω·cm~2。经XRD和俄歇能谱剖面分析结果表明,Ti和Al之间反应形成低功函数AlTi金属相可降低势垒高度,同时氮(N)的析出与Ti之间会形成低电阻低功函数的TiN金属相,导致在GaN导带边缘能级位置以n型施主态存在大量N空位,使金属与n-GaN界面处产生重掺杂效果,导致势垒宽度变薄有利于形成高质量的欧姆接触。
5、基于虚栅模型解释GaN HEMT电流崩塌效应的产生机理;为尽量降低自热效应特别设计制作了栅宽10μm的AlGaN/GaN HEMT器件进行电流崩塌实验,建立了脉冲条件下电流崩塌实验新方法。研究指出,脉冲频率和宽度变化均会导致器件呈现不同程度的电流崩塌效应,与器件表面态中电子的俘获和释放机制密切相关。并通过优化设计场板结构器件参数,从而降低了沟道电子峰值温度达到抑制电流崩塌目的。实验还发现,钝化后GaN HEMT电流崩塌量降低至仅4.7%。
6、优化设计影响频率和功率特性的器件结构和关键工艺参数,研发了有和无台面隔离的器件版图和工艺流程,制作了具有优良频率和功率性能的X波段AlGaN/GaN HEMT器件。
研究结果表明,半绝缘SiC衬底上0.25μm栅长、100μm栅宽的AlGaN/GaN HEMT器件,在零栅压下源漏饱和电流为1112mA/mm,跨导250mS/mm;截止频率fT和最大振荡频率f_(max)分别为41.5GHz和108GHz;采用负载-牵引(Load Pull)方法在栅源电压-3.2V、源漏电压28V和8GHz连续波条件下测试,获得器件的输出功率密度是5.62W/mm、增益7.49dB、功率附加效率31%。同时,总栅宽3mm的AlGaN/GaN HEMT器件经管壳封装后测试,其源漏饱和电流为2.5A,跨导660mS;在频率8GHz、栅压-2.5V和源漏电压40V偏置条件下,器件最大输出功率达15.85W,增益6.95dB,功率附加效率36%。
Wide bandgap AlGaN/GaN high-electron-mobility transistors (HEMT), which are suitable for high temperature, high frequency and high power device applications, have been regarded as the next generation technology all over the world. However, many problems of the lattice mismatch and thermal mismatch exist in AlGaN/GaN HEMT materials grown by heterogeneous epitaxial technology. Especially in wurzite GaN based transistor structures, the spontaneous polarization and piezoelectric polarization is very large, lots of mechanism of device physics effects are still ambiguous. And the related experimental studies in AlGaN/GaN HEMT devices such as characterization and strain analyse of heterostructure materials, ion implantation doping of GaN, ohmic contact and current collapse effects have not been fundamentally solved.
The work presented in this dissertation focused on the key device physics problems and related experiments of X-band AlGaN/GaN HEMTs, and the major achievements and results of the dissertation are listed as followings.
1. Based on the research of spontaneous polarization and piezoelectric polarization in AlGaN/GaN HEMTs, the relationship between the polarizability and lattice constants and the elastic coefficient is obtained in GaN-based heterojunction. With the principle of electroneutrality equilibrium, the physical model between the polarization induced sheet charge and the sheet 2DEG concentration for undoped Ga-face AlGaN/GaN HEMT structures is achieved.
2. By the high resolution X-ray diffraction (HRXRD) technology, the accurate measurement methods of lattice parameters in GaN-based materials have been established, and the horizontal and vertical strain of the GaN heterojunction is conducted by further research. By means of Williamson-Hall method, the mosaic structure parameter of the epitaxial materials is measured in order to clarify the dislocation types and calculate the dislocation density in GaN-based materials. Then the accurate calculations of the dislocation densities of screw dislocation and edge dislocation in GaN materials have been made, and the total dislocation density is about 109cm~(-2).
3. Si-ion implantation for nonalloyed contacts to an GaN heterostructure materials has been investigated, and highly-doped GaN material is obtained by means of the rapid thermal annealing(RTA) technology at relatively lower temperature (<1100℃), which offers a new technical approach for high quality ohmic contact. Results shows that the sheet electron concentration is 2×10~(15)cm~(-2) and the square resistance is 100?/□after rapid thermal annealing at 1100°C for the Si ion implantation into GaN with doses of 10~(16)cm~(-2) at energy of 100keV,and the threading dislocation densities of the samples is decreased to 1.55×10~9cm~(-2). Meanwhile, the results of photoluminescence(PL) spectra show two blue luminescence (BL) bands with energy of 2.61eV and 2.67 eV respectively, both of which differ from the previous reported results. Further studies indicate that the 2.61 eV BL band emission is attributed to an electron transition between the donor level ON and the deep acceptor complex level VGa-SiGa, and the energy of 2.67 eV BL band is considered as the transition from the shallow donor level SiGa at 20meV under the conduction band to the deep complex acceptor level VGa-ON.
4. High quality and multilayer metal ohmic contact to the unintended doped GaN with low specific contact resistivity has been achieved. Ohmic contact containing Ti(15nm)/Al(220nm)/Ni(40nm)/Au(50nm) four-layer metals have displayed the lowest contact resistivity on unintended doped GaN. After rapid thermal annealing at 900°C for 1 min with N2 ambient, the lowest contact resistivity of 1.26×10-7Ω·cm~2 is obtained. The studies of X-ray diffraction and Auger electron spectroscopy have been made to investigate the microstructure of the annealed contacts. The key to the success of the ohmic contact is the Ti layers placed between the Al layer and GaN. Upon rapid thermal annealing, there occurrs both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film, Ti reduces the native oxide on GaN and the in-diffusion of this leads to the formation of TiN when Ti reacts with GaN, a high concentration of nitrogen vacancies is created near the interface, causing the GaN to be heavily doped n-type. While the out-diffusion of this leads to the formation of low work function Ti-Al intermetallic phase. The intimate contact between the low work function intermetallic and n-GaN results in a low barrier height, allowing electron to flow in direction through the heterojunction interface by tunnel effect emission.
5. Based on virtual gate model, the physical mechanism of current collapse has been analyzed. In order to eliminate the self-heating effects of AlGaN/GaN HEMT , A special device with only 10μm gate-width is fabricated to investigate current collapse effects. A new experimental method with pulsed signal to study current collapse phenomena is established. Results shows that the variation of pulse frequency and pulse width are both induced a change of current collapse of GaN HEMTs, which is related immediately with the mechanism of electron capture and release in the surface states in the device. The optimization of the structure parameters for the field-plated GaN-based HEMT is achieved to eliminate current collapse by reducing the peak electron temperature in the device channel. The degree of current collapse of the GaN HEMTs after passivation is reduced to only 4.7%.
6. Optimization design of various structure parameters and device processes, which is influenced of characteristics of working frequency and power-output, is achieved. The key device fabrication processes, with or without mesa isolation, have been developed, and the AlGaN/GaN HEMTs with superior frequency and power characteristic have been fabricated.
The AlGaN/GaN HEMTs grown on semi-insulating 6H-SiC substrates with 0.25μm gate-length and 100μm gate-width is presented. The DC measurement results exhibit a maximum drain current density of 1112mA/mm at a zero gate voltage and the peak extrinsic transconductance of 250mS/mm. On-wafer RF measurements show the values of unity current gain cutoff frequency (fT) of 41.5GHz and maximum frequency of oscillation (f_(max)) of 108GHz is evaluated by extrapolation of the unity gain and MSG data at 20dB/decade. The device under 8GHz continuous wave conditions biased at a drain-source voltage of 28V and gate-source voltage of -3.2V shows a saturated output power of 5.62W/mm with an associated gain of 7.49dB and PAE of 31%. And the total gate periphery of 3mm AlGaN/GaN HEMTs after shell package shows a maximum drain current of 2.5A at zero gate voltage and the peak extrinsic transconductance of 660mS; Biased at drain-source voltage of 40V and gate-source voltage of -2.5V, 15.85W total power is achieved at 8GHz CW conditions, with gain of 6.95dB and PAE of 36%.
本论文围绕X波段AlGaN/GaN HEMT所涉及的器件物理和相关实验等科学技术问题开展了基础研究,取得的主要研究结论如下。
1、基于对AlGaN/GaN HEMT自发极化和压电极化效应的研究,导出了异质结中的极化强度和晶格常数及弹性系数之间的关系;利用电中性平衡原理,建立了非故意掺杂AlGaN/GaN HEMT器件结构中极化面电荷密度和2DEG之间的物理模型。
2、应用高分辨X射线衍射技术(HRXRD)的GaN材料晶格参数精确测量方法,深入研究了GaN基异质结构材料水平和垂直方向的应变情况;同时利用Williamson-Hall方法测量了外延材料的镶嵌结构参数,明确了材料中存在的位错类型,对螺位错和刃位错的位错密度进行了精确计算,获得总的位错密度为10~9cm~(-2)量级。
3、通过Si离子注入GaN材料的掺杂实验,在较低温度下(<1100°C)利用快速热退火技术实现了GaN的重掺杂目的,为制作高质量欧姆接触提供了新的技术途径。研究表明,当注入能量100keV和注入剂量10~(16)cm~(-2)的样品在1100°C快速热退火处理后可实现重掺杂,其载流子面密度为2×10~(15)cm~(-2)、方块电阻100Ω/□,而穿透位错密度降低至1.55×10~9cm~(-2)。同时,利用光致发光PL谱测试发现了能量分别为2.61eV与2.67eV蓝光发光(BL)谱线,这与前人研究的结果有所不同。进一步研究表明,2.61eV蓝光BL发光是由施主能级ON到深受主复合能级VGa-SiGa的跃迁发光产生的,而2.67eV BL带发光是由20meV左右的SiGa浅施主能级向VGa-ON复合深受主能级之间电子跃迁辐射发光所产生。
4、在非故意掺杂GaN上实现了低比接触电阻率的高质量多层金属欧姆接触。采用Ti(15nm)/Al(220nm)/Ni(40nm)/Au(50nm)四层金属在非故意掺杂GaN上进行欧姆接触实验研究,在N2气氛中经温度900°C快速热退火1min后获得了最低的比接触电阻率为1.26×10-7Ω·cm~2。经XRD和俄歇能谱剖面分析结果表明,Ti和Al之间反应形成低功函数AlTi金属相可降低势垒高度,同时氮(N)的析出与Ti之间会形成低电阻低功函数的TiN金属相,导致在GaN导带边缘能级位置以n型施主态存在大量N空位,使金属与n-GaN界面处产生重掺杂效果,导致势垒宽度变薄有利于形成高质量的欧姆接触。
5、基于虚栅模型解释GaN HEMT电流崩塌效应的产生机理;为尽量降低自热效应特别设计制作了栅宽10μm的AlGaN/GaN HEMT器件进行电流崩塌实验,建立了脉冲条件下电流崩塌实验新方法。研究指出,脉冲频率和宽度变化均会导致器件呈现不同程度的电流崩塌效应,与器件表面态中电子的俘获和释放机制密切相关。并通过优化设计场板结构器件参数,从而降低了沟道电子峰值温度达到抑制电流崩塌目的。实验还发现,钝化后GaN HEMT电流崩塌量降低至仅4.7%。
6、优化设计影响频率和功率特性的器件结构和关键工艺参数,研发了有和无台面隔离的器件版图和工艺流程,制作了具有优良频率和功率性能的X波段AlGaN/GaN HEMT器件。
研究结果表明,半绝缘SiC衬底上0.25μm栅长、100μm栅宽的AlGaN/GaN HEMT器件,在零栅压下源漏饱和电流为1112mA/mm,跨导250mS/mm;截止频率fT和最大振荡频率f_(max)分别为41.5GHz和108GHz;采用负载-牵引(Load Pull)方法在栅源电压-3.2V、源漏电压28V和8GHz连续波条件下测试,获得器件的输出功率密度是5.62W/mm、增益7.49dB、功率附加效率31%。同时,总栅宽3mm的AlGaN/GaN HEMT器件经管壳封装后测试,其源漏饱和电流为2.5A,跨导660mS;在频率8GHz、栅压-2.5V和源漏电压40V偏置条件下,器件最大输出功率达15.85W,增益6.95dB,功率附加效率36%。
Wide bandgap AlGaN/GaN high-electron-mobility transistors (HEMT), which are suitable for high temperature, high frequency and high power device applications, have been regarded as the next generation technology all over the world. However, many problems of the lattice mismatch and thermal mismatch exist in AlGaN/GaN HEMT materials grown by heterogeneous epitaxial technology. Especially in wurzite GaN based transistor structures, the spontaneous polarization and piezoelectric polarization is very large, lots of mechanism of device physics effects are still ambiguous. And the related experimental studies in AlGaN/GaN HEMT devices such as characterization and strain analyse of heterostructure materials, ion implantation doping of GaN, ohmic contact and current collapse effects have not been fundamentally solved.
The work presented in this dissertation focused on the key device physics problems and related experiments of X-band AlGaN/GaN HEMTs, and the major achievements and results of the dissertation are listed as followings.
1. Based on the research of spontaneous polarization and piezoelectric polarization in AlGaN/GaN HEMTs, the relationship between the polarizability and lattice constants and the elastic coefficient is obtained in GaN-based heterojunction. With the principle of electroneutrality equilibrium, the physical model between the polarization induced sheet charge and the sheet 2DEG concentration for undoped Ga-face AlGaN/GaN HEMT structures is achieved.
2. By the high resolution X-ray diffraction (HRXRD) technology, the accurate measurement methods of lattice parameters in GaN-based materials have been established, and the horizontal and vertical strain of the GaN heterojunction is conducted by further research. By means of Williamson-Hall method, the mosaic structure parameter of the epitaxial materials is measured in order to clarify the dislocation types and calculate the dislocation density in GaN-based materials. Then the accurate calculations of the dislocation densities of screw dislocation and edge dislocation in GaN materials have been made, and the total dislocation density is about 109cm~(-2).
3. Si-ion implantation for nonalloyed contacts to an GaN heterostructure materials has been investigated, and highly-doped GaN material is obtained by means of the rapid thermal annealing(RTA) technology at relatively lower temperature (<1100℃), which offers a new technical approach for high quality ohmic contact. Results shows that the sheet electron concentration is 2×10~(15)cm~(-2) and the square resistance is 100?/□after rapid thermal annealing at 1100°C for the Si ion implantation into GaN with doses of 10~(16)cm~(-2) at energy of 100keV,and the threading dislocation densities of the samples is decreased to 1.55×10~9cm~(-2). Meanwhile, the results of photoluminescence(PL) spectra show two blue luminescence (BL) bands with energy of 2.61eV and 2.67 eV respectively, both of which differ from the previous reported results. Further studies indicate that the 2.61 eV BL band emission is attributed to an electron transition between the donor level ON and the deep acceptor complex level VGa-SiGa, and the energy of 2.67 eV BL band is considered as the transition from the shallow donor level SiGa at 20meV under the conduction band to the deep complex acceptor level VGa-ON.
4. High quality and multilayer metal ohmic contact to the unintended doped GaN with low specific contact resistivity has been achieved. Ohmic contact containing Ti(15nm)/Al(220nm)/Ni(40nm)/Au(50nm) four-layer metals have displayed the lowest contact resistivity on unintended doped GaN. After rapid thermal annealing at 900°C for 1 min with N2 ambient, the lowest contact resistivity of 1.26×10-7Ω·cm~2 is obtained. The studies of X-ray diffraction and Auger electron spectroscopy have been made to investigate the microstructure of the annealed contacts. The key to the success of the ohmic contact is the Ti layers placed between the Al layer and GaN. Upon rapid thermal annealing, there occurrs both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film, Ti reduces the native oxide on GaN and the in-diffusion of this leads to the formation of TiN when Ti reacts with GaN, a high concentration of nitrogen vacancies is created near the interface, causing the GaN to be heavily doped n-type. While the out-diffusion of this leads to the formation of low work function Ti-Al intermetallic phase. The intimate contact between the low work function intermetallic and n-GaN results in a low barrier height, allowing electron to flow in direction through the heterojunction interface by tunnel effect emission.
5. Based on virtual gate model, the physical mechanism of current collapse has been analyzed. In order to eliminate the self-heating effects of AlGaN/GaN HEMT , A special device with only 10μm gate-width is fabricated to investigate current collapse effects. A new experimental method with pulsed signal to study current collapse phenomena is established. Results shows that the variation of pulse frequency and pulse width are both induced a change of current collapse of GaN HEMTs, which is related immediately with the mechanism of electron capture and release in the surface states in the device. The optimization of the structure parameters for the field-plated GaN-based HEMT is achieved to eliminate current collapse by reducing the peak electron temperature in the device channel. The degree of current collapse of the GaN HEMTs after passivation is reduced to only 4.7%.
6. Optimization design of various structure parameters and device processes, which is influenced of characteristics of working frequency and power-output, is achieved. The key device fabrication processes, with or without mesa isolation, have been developed, and the AlGaN/GaN HEMTs with superior frequency and power characteristic have been fabricated.
The AlGaN/GaN HEMTs grown on semi-insulating 6H-SiC substrates with 0.25μm gate-length and 100μm gate-width is presented. The DC measurement results exhibit a maximum drain current density of 1112mA/mm at a zero gate voltage and the peak extrinsic transconductance of 250mS/mm. On-wafer RF measurements show the values of unity current gain cutoff frequency (fT) of 41.5GHz and maximum frequency of oscillation (f_(max)) of 108GHz is evaluated by extrapolation of the unity gain and MSG data at 20dB/decade. The device under 8GHz continuous wave conditions biased at a drain-source voltage of 28V and gate-source voltage of -3.2V shows a saturated output power of 5.62W/mm with an associated gain of 7.49dB and PAE of 31%. And the total gate periphery of 3mm AlGaN/GaN HEMTs after shell package shows a maximum drain current of 2.5A at zero gate voltage and the peak extrinsic transconductance of 660mS; Biased at drain-source voltage of 40V and gate-source voltage of -2.5V, 15.85W total power is achieved at 8GHz CW conditions, with gain of 6.95dB and PAE of 36%.
引文
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