Thermal instability of copper gate AlGaN
Thermal instability of copper gate AlGaN/GaN HEMT on Si substrate
J.Park,K.Lee,H.-Y.Cha and K.Seo
Thermal reliability of nickel (Ni)and copper (Cu)gate AlGaN/GaN high electron mobility transistors (HEMTs)is investigated.Though the current-voltage characteristics of as-deposited Cu gate AlGaN/GaN HEMTs is superior to those of Ni gate AlGaN/GaN HEMTs,severe degradation was observed after aging at 2208C.This instability problem should be carefully taken into account in practical applications of Cu gate AlGaN/GaN HEMTs.
Introduction:AlGaN/GaN high electron mobility transistors (HEMTs)are excellent candidates for use in high power and high frequency appli-cations owing to their unique properties,such as high critical electric ?eld,high carrier concentration,and high saturation velocity [1,2].Since high breakdown voltage and low leakage current are essential to achieve high RF power density,much attention has been focused on the gate related issues,e.g.Schottky metals,?eld plates,and recessed structures [3–5].This is because the avalanche process usually occurs near the gate edge on the drain side due to the locally enhanced electric ?eld [6].Recently,it was reported that Cu gate AlGaN/GaN HEMTs had good device characteristics,especially low leakage current,which was suggested to be due to the high Schottky barrier height to AlGaN/GaN surface [7,8].In addition,Ao et al.reported that the Cu gate AlGaN/GaN HEMTs were thermally stable even after annealing at 5008C for 1h [9].However,no other studies were reported on thermal reliability of Cu gate AlGaN/GaN HEMTs.
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Fig.1Transfer characteristics and gate current characteristics of Ni gate AlGaN/GaN HEMT before and after thermal treatment
a Transfer characteristics
b Gate current characteristics
Inset:Schottky characteristics under small reverse and forward bias conditions
In this Letter,an important ?nding about the thermal instability of Cu gate AlGaN/GaN HEMTs is presented,which has not been reported so far to the best of our knowledge.AlGaN/GaN HEMTs on Si were fab-ricated using Ni and Cu gates and their thermal reliability was investigated.
Fabrication:The epitaxial layers designed in this study were grown on
a Si (111)substrate,and were composed of a 20A
?undoped-GaN capping layer,an 175A
?undoped-Al 0.26Ga 0.74N barrier layer,an 1m m undoped-GaN layer,and AlN/(Al)GaN transition layers from top to
bottom.The average sheet resistance across the 4-inch wafer was 300V /sq.Devices were fabricated using a pre-passivation process [4].
A 1200A
?SiN x ?lm was ?rst deposited as a pre-passivation layer using remote inductively coupled plasma-chemical vapour deposition and the
ohmic contact area was etched using inductively coupled plasma-reactive ion etch (ICP-RIE)with CF 4/O 2.Si/Ti/Al/Mo/Au metals were deposited for the ohmic contact and annealed using a rapid thermal annealing process at 8208C for 30s in N 2ambient.Mesa isolation was then performed using ICP-RIE with Cl 2/Ar.A 2m m-long gate footprint was patterned and then the exposed SiN x area was etched using a low damage,two-step etch process with CF 4/O 2.Another patterning process for the gate with a 1.5m m ?eld plate was carried out prior to the gate metallisation.Two gate metals were
compared in this work,i.e.Ni/Ir/Au (200/200/3600A
?)and Cu/Au (400/3600A ?).
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treatment at 220treatment at 220Fig.2Transfer characteristics and gate current characteristics of Cu gate AlGaN/GaN HEMT before and after thermal treatment
a Transfer characteristics
b Gate current characteristics
Inset:Schottky characteristics under small reverse and forward bias conditions
Experiments and discussion:Thermal aging treatment was carried out using a convection oven at a temperature controlled at 2208C.DC characteristics of both Ni and Cu gate devices were ?rst measured prior to thermal treatment and the same measurements were repeated after 10and 20h of thermal treatment.All the measurements were carried out using an Agilent 4155A semiconductor parameter analyser.The transfer characteristics of the Ni gate device before and after thermal treatment are shown in Fig.1a .The maximum transconductance of the Ni gate device was 200mS/mm and no signi?cant change was observed after thermal treatment.The gate leakage current characteristics before and after thermal treatment are shown in Fig.1b ,whereas the inset of Fig.1b is the Schottky characteristics under small reverse and forward bias conditions.The leakage current was reduced after thermal treatment by an order of magnitude under small reverse bias and by 3/4under large reverse bias.No noticeable change was observed under forward bias conditions.This phenomenon is similar to the ‘post-annealing’effects reported in [10],which was suggested to be related to the removal of shallow traps near the interface between the Ni and AlGaN/GaN surface.
A completely different behaviour with the same thermal treatment was observed for the Cu gate device.The transfer characteristics of the Cu gate device before and after thermal treatment are shown in Fig.2a .While the maximum transconductance of the Cu gate device
ELECTRONICS LETTERS 8th July 2010Vol.46No.14
before thermal treatment was similar to that of the Ni gate device,the thermal treatment caused severe degradation of the Cu gate device. The gate leakage current characteristics before and after thermal treat-ment are shown in Fig.2b.While the initial leakage current of the Cu gate device before thermal treatment was lower than that of the Ni gate device,signi?cant increase in the gate leakage current was observed after thermal treatment.However,an interesting phenomenon was observed in the Schottky characteristics under small reverse and forward bias conditions.As shown in the inset of Fig.2b,the leakage current was reduced and the forward turn-on voltage was increased after thermal treatment.The leakage current behaviour after thermal treatment under small reverse bias is completely opposite to that under large reverse bias.
Change in the Cu gate device characteristics after thermal treatment can be explained as follows.Cu is oxidised during thermal treatment forming a CuO x interface layer between the gate and AlGaN/GaN surface.It is known that Cu can be oxidised at such a low temperature [11].Now,a large fraction of the gate bias is applied to this CuO x inter-face layer rather than semiconductor layers,resulting in lower transcon-ductance and higher forward turn-on voltage.This large voltage drop across the CuO x layer also lowers the electric?eld at the AlGaN/GaN surface and thus reduces the leakage current under small reverse bias. However,when a large reverse bias is applied to the gate,signi?cant energy band bending occurs and now the Schottky barrier height is gov-erned by the CuO x layer.Since the energy bandgap of CuO x is known to be much smaller than that of AlGaN/GaN[12],the Schottky barrier height to the CuO x becomes lower than that to AlGaN/GaN,which in turn increases the gate leakage current.Such a phenomenon would not be observed in the Ni gate device because not only is the energy bandgap of NiO x larger than that of AlGaN/GaN but also NiO x cannot be formed at such a low temperature[13].
It is very important to note that Cu gate AlGaN/GaN HEMTs are ther-mally instable.Because the channel temperature of AlGaN/GaN HEMTs can exceed3008C under normal operation[14],such thermal instability will cause severe problems in practical applications.This should be carefully taken into account when utilising Cu-gate AlGaN/ GaN HEMTs.
Conclusion:Thermal reliability of Ni and Cu gate AlGaN/GaN HEMTs has been investigated.Though the Cu gate device exhibited slightly better characteristics than the Ni gate device before thermal treatment, severe degradation was observed after thermal treatment at2208C, which is suggested to be caused by CuO x formation during thermal treat-ment.Based on our experiments,it is concluded that Ni gate AlGaN/ GaN HEMTs are more suitable than Cu gate AlGaN/GaN HEMTs. Acknowledgment:This work was supported by the R&D programme of MKE(KI001693,Development of GaN power ampli?er for4G base station).
#The Institution of Engineering and Technology2010
31May2010
doi:10.1049/el.2010.1485
J.Park,K.Lee and K.Seo(Department of Electrical Engineering,Seoul National University,Seoul,Republic of Korea)
E-mail:park5610@snu.ac.kr
H.-Y.Cha(School of Electronic and Electrical Engineering,Hongik University,Seoul,Republic of Korea)References
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一种大屏幕拼接拼接缝的消除方法
一种大屏幕拼接拼接缝的消除方法 1 引言 图像镶嵌技术（mosai ）是图像融合技术的一种，一般指的是同种类型图像的融合。他把多幅具有重叠信息部分的图像衔接在一起，得到一幅完整的、范围更大的图像，并且去除其中的冗余信息。图像镶嵌技术的应用非常广泛。例如，虚拟现实中的全景图显示及遥感图像的处理等领域，都有广泛的应用。图像镶嵌的评价标准是镶嵌后得到的图像，不但具有良好的视觉效果，而且还要尽可能地保持图像光谱特征。通俗地说，就是镶嵌的图像越“无缝”，效果就越好。当然，这里的“无缝”，不是绝对意义上的，而是人眼分辨力以内的“无缝”。 一般情况下，进行图像拼接时，在拼接的边界上，不可避免地会产生拼接缝。这是因为两幅待拼接图像在灰度上的细微差别都会导致明显的拼接缝。而在实际的成像过程中，这种细微差别很难避免。因此图像镶嵌技术的难点就在于准确寻找图像之间的位置关系，并把两幅以上的图像平滑地衔接在一起，获取一幅全局的图像。本文的基本思想就是突破以往在寻找拼接线时，只要找到一个最佳拼接点，以此点做一条直线作为拼接线的不合理性，而是取一个闭值，在闭值范围内寻找出每个拼接点，把这些点连成的折线作为拼接线，进行拼接。 2 拼接缝消除的方法 传统的拼接缝消除的方法有很多，其中用得较多的方法有；中值滤波法、利用小波变换的方法、加权平均法等 2 . 1 中值滤波法消除拼接缝 中值滤波法是对接缝附近的区域进行中值滤波。对与周围灰度值差比较大的象素取与周围象素接近的值，从而消除光强的不连续性。中值滤波器处理接缝附近的狭长地带。该方法速度快，但质量一般。平滑的结果会使图像的分辨率下降，使图像细节分辨不出，产生图像模糊。 2 . 2 利用小波变换的方法消除拼接缝 小波变换方法也是目前比较常用的一种方法，他充分利用小波变换的多分辨率特性，很好地解决了拼接图像的接缝问题。其原理为：由于小波变换具有带通滤波器的性质，在不同尺度下的小波变换分量，实际上占有一定的频宽，尺度j 越大，该分量的频率越高，因此每一个小波分量所具有的频宽不大，把要拼接的两幅图像先按小波分解的方法将他们分解成不同频率的小波分量，只要分解得足够细，小波分量的频宽就能足够小。然后在不同尺度下，选取不同的拼接宽度，把2 个图像按不同尺度下的小波分量先拼接下来，然后再用恢复程序，恢复到整个图像。这样得到的图像可以很好地兼顾清晰度和光滑度2 个方面的要求。但是，小波变换也存在缺点，如小波变换的算法比较复杂，需要在小波变换域内先进行拼接处理，在计算过程中涉及到大量的浮点运算和边界处理问题，对实际生产中的大容量图像进行处理时计算机内存开销很大，且处理时间较长，拼接速度慢。 2 . 3 利用加权平滑的方法消除拼接缝
大屏幕拼接中的拼接缝消除方法
大屏幕拼接中的拼接缝消除方法 1 引言 图像镶嵌技术（mosai ）是图像融合技术的一种，一般指的是同种类型图像的融合。他把多幅具有重叠信息部分的图像衔接在一起，得到一幅完整的、范围更大的图像，并且去除其中的冗余信息。图像镶嵌技术的应用非常广泛。例如，虚拟现实中的全景图显示及遥感图像的处理等领域，都有广泛的应用。图像镶嵌的评价标准是镶嵌后得到的图像，不但具有良好的视觉效果，而且还要尽可能地保持图像光谱特征。通俗地说，就是镶嵌的图像越“无缝”，效果就越好。当然，这里的“无缝”，不是绝对意义上的，而是人眼分辨力以内的“无缝”。 一般情况下，进行图像拼接时，在拼接的边界上，不可避免地会产生拼接缝。这是因为两幅待拼接图像在灰度上的细微差别都会导致明显的拼接缝。而在实际的成像过程中，这种细微差别很难避免。因此图像镶嵌技术的难点就在于准确寻找图像之间的位置关系，并把两幅以上的图像平滑地衔接在一起，获取一幅全局的图像。本文的基本思想就是突破以往在寻找拼接线时，只要找到一个最佳拼接点，以此点做一条直线作为拼接线的不合理性，而是取一个闭值，在闭值范围内寻找出每个拼接点，把这些点连成的折线作为拼接线，进行拼接。 2 拼接缝消除的方法 传统的拼接缝消除的方法有很多，其中用得较多的方法有；中值滤波法、利用小波变换的方法、加权平均法等。 2.1 中值滤波法消除拼接缝 中值滤波法是对接缝附近的区域进行中值滤波。对与周围灰度值差比较大的象素取与周围象素接近的值，从而消除光强的不连续性。中值滤波器处理接缝附近的狭长地带。该方法速度快，但质量一般。平滑的结果会使图像的分辨率下降，使图像细节分辨不出，产生图像模糊。 2.2 利用小波变换的方法消除拼接缝 小波变换方法也是目前比较常用的一种方法，他充分利用小波变换的多分辨率特性，很好地解决了拼接图像的接缝问题。其原理为：由于小波变换具有带通滤波器的性质，在不同尺度下的小波变换分量，实际上占有一定的频宽，尺度j 越大，该分量的频率越高，因此每一个小波分量所具有的频宽不大，把要拼接的两幅图像先按小波分解的方法将他们分解成不同频率的小波分量，只要分解得足够细，小波分量的频宽就能足够小。然后在不同尺度下，选取不同的拼接
浅谈如何能够消除液晶拼接屏拼缝困扰
浅谈如何能够消除液晶拼接屏拼缝困扰 一直以来拼缝一直是困扰液晶拼接发展的一个重要因素，其实想要消除液晶拼接屏拼缝的方法有很多，下面向大家介绍三种最主要的方法：加权平均法、利用小波变换的方法、中值滤波法等。 一、加权平滑的方法消除拼接缝 在实际中，使用比较多的方法还是对重叠区域进行加权平滑的方法。这种方法的思路是：图像重叠区域中象素点的灰度值Pixel由两幅图像中对应点的灰度值LPixel和RPixel加权平均得到，即：Pixel一kXLPixel+(l一k)XRPixel 其中：k是渐变因子，满足条件：o 寻找最佳拼接线时，采用一个滑动窗口在图像重叠区上逐行选择灰度值差异最小的象元作为最佳拼接点。但是，如果按照这种拼接点选择法，会出现一个新问题，就是往往会出现上下行拼接点位置相差较大的现象，这样拼接后有时因上下行之间灰度差异较大而造成新的接缝。为避免这类现象发生，不仅要考虑相邻拼接点的灰度值差异，而且还要考虑相邻拼接点的位置不能太远。 这样就引进了一个阑值T，把选择最佳拼接点的范围限制在这个阑值内。除第一行按灰度值差异最小的原则处理外，其他各行的拼接点从一个选定区
域中选取：即与上一行所选拼接点同列的点及以该点为中心左右宽度为T的区域中的点。 在这个区域中选取一个最佳拼接点。选出每行的拼接点后连接成一条拼接线，可想而知，这条拼接线可能是条折线。这样，由于各行都是选择规定邻域内灰度差异最小的点作为拼接点，接缝现象就会得到很大的改观。同时，T的值又不能选取得太大，应在1一5之间选取为佳。找出最佳拼接缝后，按前面所述的加权平滑对重叠区域再进行过渡，得到的图像质量有很大改观 二、小波变换的方法消除拼接缝 小波变换方法也是目前较为常用的一种方法，他充分利用小波变换的多分辨率特性，很好地解决了拼接图像的接缝问题。其原理为：由于小波变换具有带通滤波器的性质，在不同尺度下的小波变换分量，实际上占有一定的频宽，尺度j越大，该分量的频率越高，因此每一个小波分量所具有的频宽不大，把要拼接的两幅图像先按小波分解的方法将他们分解成不同频率的小波分量，只要分解得足够细，小波分量的频宽就能足够小。然后在不同尺度下，选取不同的拼接宽度，把2个图像按不同尺度下的小波分量先拼接下来，然后再用恢复程序，恢复到整个图像。这样得到的图像可以很好地兼顾清晰度和光滑度2个方面的要求。但是，小波变换也存在缺点，如小波变换的算法比较复杂，需要在小波变换域内先进行拼接处理，在计算过程中涉及到大量的浮点运算和边界处理问题，对实际生产中的大容量图像进行处理时计算机内存开销很大，且处理时间较长，拼接速度慢。