【作者】 刘艳；

【导师】 林兆军；

【作者基本信息】 山东大学 ， 微电子学与固体电子学， 2017， 博士

【摘要】 信息化已成为当今社会发展的大趋势,微电子技术在推进社会信息化进程中起了很大作用。而半导体器件性能的提高直接影响了微电子技术的发展。半导体器件在以高效率、小型化等为发展目标不断追求技术进步的同时,也越来越注重追求低污染、低能耗的环保优点。采用新兴半导体材料与器件实现高效小型及节能环保刻不容缓。其中,作为GaN基电子器件重要代表的AlGaN/GaN异质结场效应晶体管(AlGaN/GaNHFETs),因其优异的高温高频大功率等特性能够满足当今社会发展对半导体器件高性能的要求,而备受国内外的广泛关注。早在2010年,商用AlGaN/GaN HFETs器件就已推向市场。近年来,随着GaN技术的迅速发展,高性能商用AlGaN/GaN HFETs产品不断更新并已在越来越多领域实现应用。其中,高温领域的应用是AlGaN/GaNHFETs的重要优势之一。因此对器件特性随温度变化的研究非常必要。目前有关AlGaN/GaN HFETs器件变温特性的报道集中在对高温下其饱和漏电流密度、跨导和截止频率等的下降上即其电学特性退化的评估上,而缺乏对其退化机理的深入探讨。明确器件性能参数随温度变化的规律和机理是实现器件在高温下保持高性能的基础。电子迁移率是研究器件性能参数的重要参数。而散射机制是电子迁移率的重要影响因素。因此研究散射机制随温度变化的规律对分析器件的性能参数受温度的影响至关重要。2007年,在对AlGaN/GaN HFETs器件输运特性的研究中,一种新散射机制——极化库仑场散射(PCF散射)被首次提出。PCF散射源于AlGaN/GaN异质结界面处分布不均匀的极化电荷。器件制造过程中金属淀积工艺及器件工作过程中漏极和栅极所加电压导致源漏间AlGaN势垒层应变分布不均匀是引起异质结界面处极化电荷分布不均匀的主要原因。从2007年PCF散射机制被首次提出,到2014年这一新散射机制理论模型建立,极化库仑场散射理论在不断完善中已历经近10年发展。期间直至目前理论验证和修正的研究工作一直在进行。然而这些研究大都仅限于室温下PCF散射对GaN电子器件载流子输运特性和器件电学特性的影响。变温情况下PCF散射对GaN电子器件载流子输运特性和器件电学特性的影响尚需研究和探讨。因此本论文主要就温度对AlGaN/AlN/GaN异质结场效应晶体管(AlGaN/AlN/GaN HFETs)中二维电子气(2DEG)电子迁移率(μn)和器件电学参数的影响进行了研究(目前的AlGaN/GaNHFETs器件都是采用了有薄A1N插层的AlGaN/AlN/GaN HFETs器件,本论文研究的AlGaN/GaN HFETs器件,都是AlGaN/AlN/GaN HFETs器件),特别是温度变化过程中,就PCF散射对AlGaN/AlN/GaN HFETs器件中μn的影响进行了研究,进而分析研究了高温过程中,PCF散射对AlGaN/AlN/GaN HFETs器件的性能参数Rs(栅源间寄生电阻)的影响情况。具体包括以下内容:1、AlGaN/AlN/GaN HFETs中2DEG电子体系的变温研究制备了源漏间距100μm,栅长30μm的AlGaN/AlN/GaN HFETs器件,对制备的AlGaN/AlN/GaN HFETs器件进行了变温电流-电压(I-V)和电容-电压(C-V)测试,基于测试得到的不同温度下的C-V和I-V曲线,并通过在Matlab环境下薛定谔方程和泊松方程的自洽求解,研究了温度对AlGaN/AlN/GaN HFETs中2DEG电子体系(包括2DEG的浓度、子带结构、子带占据和三角形势阱深度等)的影响。发现AlGaN/AlN/GaN异质结界面处的三角形势阱随温度的升高而逐渐变浅变宽,对2DEG电子的限制作用也随之逐渐变弱,导致2DEG电子分布逐渐延展向GaN一侧;且栅偏压为零时的2DEG电子密度、费米能级Ef和异质界面极化电荷密度随之逐渐减小。这其中一个原因是随温度上升,电子热激发能(KBT)增大,三角形势阱中的2DEG电子热激发到更高能态的几率增大,2DEG电子体系的量子特征减弱;另外一个重要原因归为AlGaN势垒层及GaN沟道层具有的热膨胀系数不同,因此升高温度,将减小AlGaN势垒层的应变能,从而会减弱AlGaN势垒层的极化电场,进而将变浅三角形势阱的阱深,最终降低了2DEG电子的限制作用。2、AlGaN/AlN/GaN HFETs中2DEG电子迁移率的变温研究(a)室温下AlGaN/AlN/GaN HFETs中2DEG电子迁移率的研究。制备了源漏间距为100μm、栅长为40μm的AlGaN/AlN/GaN HFETs器件,并通过实验测得的AlGaN/AlN/GaNHFETs的C-V、I-V特性曲线,结合理论计算定量研究了室温下各种散射机制(长纵光学声子散射(LO声子散射)、界面粗糙度散射(IFR散射)、声学形变势散射(DP散射)、压电散射(PE散射)、位错散射(DIS散射)以及PCF散射)对AlGaN/AlN/GaN HFETs中栅下沟道2DEG电子迁移率的影响。通过μn随2DEG面密度的变化进行分析表明:室温下LO声子散射对μn的大小起主导作用,而PCF散射对μn随2DEG面密度的变化趋势起主导作用。(b)AlGaN/AlN/GaN HFETs中2DEG电子迁移率的低温研究。通过对所制备的器件(低温研究器件与室温研究器件相比,除在异质结材料体系上有所不同外,器件尺寸和器件工艺等方面都一样)在100～300K范围内进行C-V和I-V曲线测试,并应用马西森定则定量分析了低温100-300K下各散射机制对AlGaN/AlN/GaN HFETs中2DEG电子迁移率的影响。通过对低温100～300K范围内μn随2DEG面密度变化情况的分析表明:100～200K温度范围,PCF散射是栅下沟道载流子最重要的散射机制,PCF散射对栅下沟道中μn的大小及μn随2DEG面密度变化的趋势都起主导作用;随着温度的升高PCF散射对μn的影响不断减弱,出现这一结果的主要原因为:温度升高过程中LO声子散射不断增强,导致PCF散射对μn影响的比重不断减小。(c)不同器件结构的AlGaN/AlN/GaN HFETs中2DEG电子迁移率的高温研究。制备了源漏间距均为100μm、栅长分别为30μm、50μm及70μm的三个方形AlGaN/AlN/GaN HFETs样品。通过高温300～500K范围内测试得到的C-V、I-V特性曲线,分析了高温300～500K下的三个样品中μn随2DEG面密度的变化情况。研究表明:在300～500K温度范围内,PCF散射依然是AlGaN/AlN/GaNHFETs器件中栅下沟道载流子的一种重要散射机制,特别是对LGG/LSD较小的器件(LG/LSD<1/2),即使在温度上升到500K时,PCF散射依然对栅下沟道中μn随2DEG面密度的变化趋势有重要影响。3、AlGaN/AlN/GaN HFETs中栅源间电阻Rs的高温研究(a)温度对栅源间电阻Rs的影响。制备了源漏间距、栅源间距及栅长分别为100μm、20μm及20μm的方形AGaN/AlN/GaN HFETs器件,并通过高温300～500K范围内测得的I-V、C-V特性曲线及栅探针法测得的Rs,分析研究了温度对AlGaN/AlN/GaN HFETs中Rs的影响,特别是就不同温度下PCF散射对Rs的影响进行了研究。研究表明:在300～500K温度范围内,PCF散射对AlGaN/AlN/GaN HFETs中的Rs有重要影响;随温度增加,在同样正向栅源电流条件下,栅源偏压降低,栅下附加极化电荷量减小,其对应的PCF散射势减弱,从而使得PCF散射对Rs的影响降低;通过对AlGaN/AlN/GaN HFETs中Rs的变温分析证实了栅下附加极化电荷对应的附加散射势(PCF散射势)与欧姆接触附近附加极化电荷对应的附加散射势(PCF散射势)在散射AlGaN/AlN/GaN HFETs器件栅源间沟道载流子时存在相互作用。(b)不同器件结构的AlGaN/AlN/GaN HFETs中栅源间电阻Rs的高温研究。制备了源漏间距和栅长均为100μm和20μm,栅源间距分别为10μm及30μm的两个方形AlGaN/AlN/GaN HFETs样品。通过高温300～500K范围内测试得到的C-V、I-V特性曲线及栅探针法测得的Rs,分析研究了不同器件结构的AlGaN/AlN/GaNHFETs中Rs的高温特性。研究表明:对于栅长相同而栅源距离不同的AlGaN/AlN/GaNHFETs器件,栅源距离越小,PCF散射对该器件中的Rs影响越大,这为优化AlGaN/AlN/GaN HFETs器件结构,减小寄生Rs电阻提供了重要依据。更多还原

【Abstract】 The promotion of information technology is trend in the development of the society in today’s age.The microelectronics technology has a great impact on the process of the information society.For the development in microelectronics technology,it is directly influenced by the improvement of the performance of the semiconductor devices.Efforts are always being put into reducing the size of the electronic product and mounting to increase the power delivery efficiency.At the same time,people are deeply concerned about whether the electronic products can have the advantages of lowering the energy consumption and pollution emission or do not.The adoption of the emerging semiconductor materials and devices is of great urgency to make electronic products high efficiency,miniaturization,and energy conservation.Due to its excellent properties,such as high temperature,high frequency,and high power microwave and so on,AlGaN/GaN heterojunction field-effect transistors(AlGaN/GaN HFETs),as an important branch of the GaN-based electronics devices,can meet solve the puzzle as mentioned above.Thus,it arouses broad interest at home and abroad.Early in 2010,the AlGaN/GaN HFETs devices utilized commercially have emerged.In recent years,many new types of AlGaN/GaN HFETs devices have been released and applied in more and more fields.High temperature application is one of the important advantages for AlGaN/GaN HFETs.Hence,it is necessary to study the temperature-dependent performance of the devices.So far,there are many researches on temperature-dependent characteristics.However,in the most of these researches many attentions are paid to the evaluation on the degradation of the electrical properties such as the drain-to-source current,transconductance and cut-off frequency,and less attentions to the study on the mechanism of the degradation.Therefore,it is necessary to pay close attention to this problem.It is helpful to improve the performance of the device to make clear the mechanism about the effect of temperature on the performance of the devices.In GaN HFETs,the 2DEG electron mobility is an important parameter for studying the performance of the devices.Scattering mechanism is an important influence factor for electrom mobility.Therefore,the study on the temperature-dependent scattering mechanism is of great importance.In 2007,during the study of the transport mechanism in AlGaN/GaN HFETs,a new kind of scattering mechanism i.e.Polarization Coulomb field scattering(PCF scattering)was first proposed.PCF scattering is originated from the uneven polarization charge distribution along the AlGaN/GaN heterojunction interface.Each process of the device manufacturing and the gate-source or drain-source voltage can have some influences on the uniformity of the strain for the source-to-drain AlGaN barrier layer.Nonuniformity of the strain can lead to the nonuniformity of the polarization charges,which could scatter the 2DEG in the channel of the devices.From 2007 when the polarization Coulomb field scattering was first proposed,to 2014 when the theoretical model of such a new kind of the scattering mechanism was established,PCF scattering has been studying for about 10 years.Even now,the research on the validation and adjustment about theory is making.However,those researches were all made at room temperature.In fact,the influence of PCF scattering on the carrier transport of GaN electronic devices and the electrical properties at different temperatures still needs further study and discussion.Therefore,the main work in this dissertation is to study the influence of the temperature on the 2DEG electron mobility for AlGaN/AlN/GaN heterojunction field-effect transistors(AlGaN/AlN/GaN HFETs)(So far,AlGaN/AlN/GaN HFETs device with the thin AIN intercalation is always adopted for AlGaN/GaN HFETs,therefore the study on the AlGaN/GaN HFETs in this paper are all AlGaN/AlN/GaN HFETs),especially the influence on the PCFs cattering.And the discussions on the influence of PCF scattering on the gate-to-source resistance Rs are further made in the high temperature range.The main conclusions of the dissertation are listed below:1.Temperature-dependent 2DEG electronic system in the AlGaN/AlN/GaN HFETsAlGaN/AlN/GaN HFETs with the drain-to-source length being 100μm and the gate length being 30μm has been prepared.And I-V and C-V measurements for the prepared AlGaN/AlN/GaN HFETs were performed at different temperatures.With the measured temperature-dependent I-V and C-V curves,and by self-consistently solving Schrodinger equation and Poisson equation in MATLAB environment,the influence of temperature on 2DEG electronic system in the AlGaN/AlN/GaN HFETs,involving 2DEG sheet density,the subband structure,the subband occupation and the depth of the triangle quantum well,were investigated.It is shown that as the temperature increases,the triangular quantum well becomes wider and therefore the distribution of the 2DEG electron spreads deep into GaN;and the 2DEG electron sheet density corresponding to the zero gate voltage,the Fermi level,and the polarization of the AlGaN barrier layer at heterojunction interface all decreases.There are two mainly reasons.One reason is that as the temperature increases,the thermal excitation-energy(KBT)of electron could increase.As a result the electron excitation probability for the 2DEG electron in the triangular quantum well would also increase and the quantum characteristics of 2DEG electronic system becomes not obvious.The other reason can be attributed to the difference in thermal expansion coefficient between AlGaN and GaN,which reduces the strain energy and the porlarization electron field in the AlGaN barrier layer.2.Temperature-dependent 2DEG electron mobility in the AlGaN/AlN/GaN HFETs(a)The study on the 2DEG electron mobility in the AlGaN/AlN/GaN HFETs at room temperature.The AlGaN/AlN/GaN HFET with the drain-to-source length being 100μm and the gate length being 40μm has been prepared,and with the measured temperature-dependent I-V and C-V curves,and the theoretical calculation,the relationship between the electron mobility under the gate and the different kinds of scattering mechanisms which include PCF scattering,longitudinal optical(LO)phonon scatterinig,interface roughness(IFR)scattering,dislocation(DIS)scattering,acoustic deformation potential(DP)scattering,and piezoelectric(PE)scattering has also been analyzed and studied quantitatively at room temperature.It is found that the value of the carrier mobility is mainly determined by LO phonon scattering,and the change trend for the carrier mobility with respect to the 2DEG density is mainly determined by PCF scattering.(b)The study on the 2DEG electron mobility in the AlGaN/AlN/GaN HFETs at low temperature.With the measured I-V and C-V curves in the temperature range 100-300K for the fabricated device(The device used in the study at low temperature is the same as that at room temperature except for the heterojunction material system),and combining with Matthiessen’s law,the influence of all kinds of scattering mechanisms on 2DEG electron mobility in the AlGaN/AlN/GaN HFETs was investigated.By analyzing the varation of the 2DEG electron mobility with 2DEG sheet density,it is found that in the temperature range 100-200K,PCF scattering plays a more significant role in the impact on the mobility of the carrier in the channel under the gate,and the value of the carrier mobility and the change trend for the carrier mobility with respect to the 2DEG density is mainly determined by PCF scattering.And the role in the impact of PCF scattering on the electron mobility will weaken with increasing temperature.The reason can be attributed to the stronger LOphonon scattering with increasing temperature which makes PCF scattering be relatively weaker in affecting the electron mobility.(c)The influence of the device structure on high temperature 2DEG electron mobility in AlGaN/AlN/GaN HFETs.The AlGaN/AlN/GaN HFETs with the different the drain-to-source length and the gate length(100pm/30μm,100μm/50μm,and 100μm/70μm)were prepared.With the measured I-V and C-V curves in the temperature range 300-500K,the relationship between the electron mobility and 2DEG sheet density at different temperatures were studied.It is shown that in the temperature range 300-500K,PCF scattering still has an important effect on the change trend for the electron mobility with respect to the 2DEG density,and PCF scattering still plays a more significant role in the impact on the mobility of the carrier in the channel under the gate.And particularly for the devices with the samller ratio of gate length to drain-to-source distance(LG/LSD<1/2),even the temperature is up to 500K,the change trend for the carrier mobility with respect to the 2DEG density is still mainly determined by PCF scattering.3.High-temperature-dependent the source-to-gate resistance Rs in the AlGaN/AlN/GaN HFETs(a)The influence of the temperature on the source-to-gate resistance Rs.The AlGaN/AlN/GaN HFET with the drain-to-source length being 100μm and the gate length being 20μm has been prepared,and using the measured temperature-dependent Rs and both C-V and I-V characteristics for the prepared device,the influence of temperature on Rs in the AlGaN/AlN/GaN HFETs was investigated.In particular,the influence of PCF scattering on Rs in the AlGaN/AlN/GaN HFET has been investigated at elevated temperatures.It is found that in the temperature range 300-500K,PCF scattering has an important influence on the parasitic source resistance Rs.And at the same the forward gate-to-source current,the gate-to-soure bias would decrease and so would the magnitude of the additional polarization charge under the gate.As a result the potential of the PCF scattering and the impact of PCF scattering on the Rs would weaken with increasing temperature.And the interaction between the positive additional polarization charges underneath the gate contact and the negative additional polarization charges near the source contact was verified during the variable-temperature study of Rs.(b)The influence of the device structure on the gate-to-source resistance Rs in AlGaN/AlN/GaN HFETs at high temperature.Two AlGaN/AlN/GaN HFETs with the same drain-to-source,the same gate length,and the different gate-to-source length(100μm/20μ/10μm,and 100μm/20μm/30μm)were prepared.With the measured temperature-dependent Rs and both C-V and I-V characteristics for the prepared devices,the influence of the device structure on Rs in the AlGaN/AlN/GaN HFETs has been investigated at elevated temperatures.It is found that for the AlGaN/AlN/GaN HFETs devices with the same gate length and the different gate-to-source distance,the shorter the gate-to-source distance,the more influence on the parasitic source resistance Rs the PCF scattering has.This finding is of great benefit to obtain optimized performance of the AlGaN/AlN/GaN HFETs and reduce.the parasitic source resistance Rs.更多还原