New STM technique for observation of ballistic electrons in semiconductors

我们提出并证明了一种新的STM技术——扫描热电子显微镜（SHEM），它通过STM探针检测向固体材料表面传播的弹道热电子（HE），测量HE的空间分布和能谱。为了将尖端和表面之间的电势峰值降低到HE能量，我们推导了尖端和表面的功函数、隧道电压和尖端-表面间距所需的理论关系。我们设计了一个实验，在大气环境中验证了我们的想法。我们选择了Si/CaF2/Au异质结构，自洽地分析了偏置电压施加下的能带分布，表明在10KA/cm2的电流密度下有可能产生能量为3eV的热电子。我们利用光刻技术制作了面积为10um2的热电子发射器，以获得设计的发射特性。我们测量了在向发射极施加3V的直流电压、频率为82Hz的交流电压170mV以及1.5V的隧道电压的情况下，针尖接近表面的相敏检测（PSD）输出。当针尖接近样品一定距离时，输出突然增加，显示出热电子检测，这是扫描探针针尖首次检测到HE。然而，集成需要相当长的时间才能获取一个数据点来对抗噪声。我们从理论上获得了针对尖端振动抗扰度的最佳检测条件。我们开发了隧道电流的数字记录和处理系统来分析噪声特征并提取HE信号。从功率谱分析和上述理论可以清楚地看出，PSD可以在10秒的积分时间下检测到1pA HE信号。通过这项研究，我们已经建立了获得SHEM系统数据的技术。 HE空间分布的测量和半导体中热电子的检测已成为下一阶段的目标。较少的

We have proposed and proved a new STM technique, Scanning Hot Electron Microscopy (SHEM), which has detected ballistic hot electrons (HE) propagating towards a surface of a solid material by a probe of STM to measure a spatial distribution and an energy spectrum of the HE.To lower the potential peak between the tip and the surface down to the HE energy, we have derived theoretical relation required for the work functions of the tip and the surface, the tunnel voltage and the tip-surface spacing. We have designed an experiment which has verified our idea in an atmospheric environment. We have selected Si/CaF2/Au heterostructure, analyzed self-consistently a band profile under the bias voltage application to show a possibility of generation of the hot electron with energy of 3eV at current density of 10KA/cm2. We have fabricated hot electron emitters with the area of 10um2 by using photolithography to obtain emission characteristics as designed. We have measured the phase sensitive detec … More tion (PSD) output approaching of the tip to the surface under application of DC voltage of 3V,AC voltage of 170mV at frequency of 82Hz to the emitter and the tunnel voltage of 1.5V.When the tip approached to the sample within certain distance, the output increased suddenly to show the hot electron detection, which was the first detection of the HE by the scanning probe tip. However, it took quite long time for integration to take one data point against the noise. We have theoretically obtained the optimum condition for the detection with respect to the immunity for the tip vibration. We have developed digital recording and processing system of the tunnel current to analyze the noise characteristics and to extract HE signal. It has been made clear from the power spectrum analysis and the above theory that 1pA HE signal can be detected by the PSD at integration time of 10sec. We have established the technique to obtain systematic data of SHEM through this research. The measurement of the spatial distribution of HE and the detection of the hot electron in the semiconductor has been left as the next stage targets. Less