Fabrication and characteristics of excellent current spreading GaN-based LED by using transparent electrode-insulator-semiconductor structure

    Corresponding author: Chenglin Qi, qichenglin@semi.ac.cn
  • Semiconductor Lighting Technology Research and Development Center, Institute of Semiconductors, State Key Laboratory of Solid State Lighting, Beijing Engineering Research Center for the 3rd Generation Semiconductor Materials and Application Chinese Academy of Sciences, Beijing 100083, China

Key words: electrode-insulator-semiconductor (EIS)light emitting diodes (LEDs)tunneling mechanismluminous uniformitycurrent spreading

Abstract: GaN-based vertical light-emitting-diodes (V-LEDs) with an improved current injection pattern were fabricated and a novel current injection pattern of LEDs which consists of electrode-insulator-semiconductor (EIS) structure was proposed. The EIS structure was achieved by an insulator layer (20-nm Ta2O5) deposited between the p-GaN and the ITO layer. This kind of EIS structure works through a defect-assisted tunneling mechanism to realize current injection and obtains a uniform current distribution on the chip surface, thus greatly improving the current spreading ability of LEDs. The appearance of this novel current injection pattern of V-LEDs will subvert the impression of the conventional LEDs structure, including simplifying the chip manufacture technology and reducing the chip cost. Under a current density of 2, 5, 10, and 25 A/cm2, the luminous uniformity was better than conventional structure LEDs. The standard deviation of power density distribution in light distribution was 0.028, which was much smaller than that of conventional structure LEDs and illustrated a huge advantage on the current spreading ability of EIS-LEDs.

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1.   Introduction
  • Over the past decades, the GaN-based light-emitting-diodes (LEDs) have been widely used as efficient light sources in solid-state lighting. They have various advantages over the traditional luminescent devices such as high energy efficiency, long life time, high color rendering index, high reliability, less response time, are environmentally friendly and have a wide scope of applications[1-3]. However, the lateral LEDs (L-LEDs) suffered from low light extraction efficiency (LEE), poor internal quantum efficiency (IQE) and serious luminous efficiency droop attributed to the poor current spreading, which may lead to reliability problems and limited applications of LEDs at high current injection situations and thus receive a great deal of research interest[4-6]. In general, vertical LEDs (V-LEDs) on copper substrate are preferred over lateral LEDs due to the better current spreading and the uniform current distribution[7]. Nevertheless, current crowding still plays a vital role in the efficiency droop on account of the extensive usage of the finger electrodes, which limits the device performance. Current accumulation near the electrodes leads to a non-uniform current distribution across the device as well as low emission efficiency that result from the poor transparency of the finger electrodes[8].

    In order to minimize the current crowding effect, highly transparent conductive layers free from finger electrodes for better current spreading are required to achieve better current spreading across the n-GaN surface[9]. In the past few years, novel strategies were provided for uniform current injection and distribution across a large device area and some wide-bandgap materials such as SiN$_x$ were introduced in the fabrication of highly transparent conductive electrodes, which demonstrate the tremendous potential of wide-bandgap materials for use in high-performance optoelectronic devices[10-12]. In this letter, a novel current injection pattern of LEDs which consists of electrode-insulator-semiconductor (EIS structure) was proposed. This EIS structure obtains a uniform current distribution on the chip surface through defect-assisted tunneling mechanism, thus greatly improving the current spreading ability of LEDs[13, 14]. The EIS-LEDs based on the V-LEDs with size of 750 $\times$ 750 $\mu $m$^{\mathrm{2}}$ were manufactured and the current-voltage ($I$-$V$) properties of the EIS-LEDs were also investigated. The $I$-$V$ curves of the EIS-LEDs showed unique properties compared to the L-LEDs and V-LEDs, and the CCD images also showed superb optical emission uniformity under different current densities.

2.   Experimental section
  • The LED epilayers used in this study were grown on $c$-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The LED structure consists of an undoped GaN layer, a Si-doped n-type GaN (n-GaN) layer, the InGaN/GaN multiple quantum wells with the electroluminescence wavelength of 460 nm, a Mg-doped AlGaN electron blocking layer, and a Mg-doped p-GaN layer. The vertical light-emitting-diodes (V-LEDs) with EIS structure were fabricated with this epitaxial wafer according to the process flow illustrated in Fig. 1. First, the Ni/Ag/Pt/Au(5/4000/500/3000 Å) contact was formed by electron beam evaporation to interconnect the individual LED cells into a whole chip as well as to act as a highly reflective layer[15-18]. Before making the metal interconnect layer, an isolation layer of SiO$_{\mathrm{2}}$ was deposited on the sidewall by plasma enhanced chemical vapor deposition (PECVD) to serve as an isolation and passivation layer. Then, a 150-$\mu$m-thick copper layer was deposited onto the samples by plating, which was used as a metal substrate. Subsequently, the sapphire substrate on the n-GaN side was removed by the KrF gas laser operating at 248 nm. Afterward, the n-type GaN layer was exposed by a Cl$_{\mathrm{2}}$-based plasma etching process with an inductively coupled plasma (ICP) etcher to obtain a low roughness surface, of which the fluctuation was $ < \pm 10$ nm. Considering that the thickness of the insulating layer was less than 50 nm, it was crucial for the n-GaN to have a low roughness surface. Figs. 2(a) and 2(b) show the scanning electron microscopy (SEM) images of the n-GaN surface before and after the ICP etching (performed at 300 W, 8 mTorr, Cl$_{\mathrm{2}}$ 15 sccm, BCl$_{\mathrm{3}}$ 30 sccm, Ar$_{\mathrm{2}}$ 5 sccm), respectively. The Root-Mean-Square (RMS) value of the surface roughness is as low as 3.3 nm after the ICP etching, which was 60 nm before the treatment. The smooth surface of the n-GaN is crucial for the sputtering of Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ in order to obtain a uniform and solid insulating layer[19]. By comparing the photoelectric properties and preparation methods of various insulating layers, and optimizing the insulating layer thickness, a 20 nm matched optimal thickness of Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ insulating layer was obtained. Above the insulating layer was an ITO transparent conductive layer (280 nm TCL) that acted as a current spreading layer. Figs. 2(c) and 2(d) shows the scanning electron microscopy (SEM) image of the details of EIS structure. As shown in Figs. 2(c) and 2(d), the sandwich structure consists of an ITO layer (for current spread in an individual LED cell), an insulation layer of Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ (as a dielectric between ITO and n-GaN) and n-GaN layer. Here, the insulation layer is used on the surface of n-GaN in order to obtain better current spreading in each LED cell as well as to avoid an electrical short of the p and n region and to guarantee minimum light absorption by the Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ layer. Finally, we successfully manufactured GaN-based blue EIS-LED with size of 750 $\times$ 750 $\mu $m$^{\mathrm{2}}$.

3.   Results and discussion
  • To start with, the current injection of the MIS structure under low bias and high bias are illustrated in Fig. 3. In order to demonstrate the function of the insulating layer and the distribution of the injection current between the ITO layer and the p-GaN, we brought out the RT and RL to show the transverse electrical resistance and the longitudinal electrical resistance, respectively. Under the low bias, the RL was cut off on account of the poor electrical conductivity of the Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ layer and the RT value was small due to the good electrical property of the ITO layer. On this condition, the electrons were distributed on the surface of the ITO near the insulating layer uniformly, which was shown in Fig. 3(a). With the raising of the externally applied bias, the electron channel would come into being in the insulating layer and the device would show a better current distribution property, which could be seen in Fig. 3(b). To verify the effectiveness of the EIS structure, the current-voltage ($I$-$V$) properties of the EIS-LED were also investigated and the schematic diagram qualitatively correlates well with the $I$-$V$ properties. As shown in Fig. 3(c), the LED sample with EIS structure was tested under a linear sweep scan 7 times and the scan voltage was maintained at 10 V for each time. The EIS-LED showed an extremely high threshold voltage of 8.5 V for the first sweep, which should be attributed to the poor electrical conductivity of Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$. However, the $I$-$V$ curve switched from the high resistance status (HRS) to the low resistance status (LRS) with the threshold voltage of 3.2 V in the second sweep. We also found that the $I$-$V$ curve shifted upward as the sweep process moved on and the threshold voltage remained unchanged. The $I$-$V$ curves of the EIS-LED under the externally applied voltage of 5, 8, 9.5, 10, 11 and 12 V are shown in Fig. 3(d). It can be seen that the $I$-$V$ curves show a HRS property with the threshold voltage of over 8 V under the voltage of 5, 8, 9.5 and 10 V. However, when the voltage goes up to 11 and 12 V, the $I$-$V$ curves switch to the LRS with the threshold voltage of 3 V approximately, which is similar to that of the traditional vertical LED. The transition from HRS to the LRS indicates that some conducting paths were formed in the Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ layer with the raising of the externally applied bias. However, the $I$-$V$ properties of the device will recover to the original state and show the same characteristic as before when the EIS-LED is put aside for 3 days or more. It was indicated that the unique property of the $I$-$V$ curve and the electrical path in the Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ layer were associated with the active O-deficient regions in the EIS structure. When the externally applied bias was high enough, the electrons can gain enough energy from the electric field to release a bound electron from the O atom of the lattice by a mechanism of impact ionization. The O vacancies acting as conducting paths were generated in the Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ layer after the first sweep of high voltage and the $I$-$V$ curve of the EIS structure switched from the HRS to the LRS.

    The conducting paths associated with the O vacancies would not be dismissed immediately when the externally applied bias was gone, which help from the formation of the electric pass in the next sweep. At the same time, the O vacancies could be oxidized gradually by the oxygen in the air and the LRS could only be maintained for a couple of days before it turned back to the HRS.

    Another important observation of Fig. 3(d) should be drawn to the threshold voltage under the HRS. For the P-N junction, the transmission probability ($P$) in the tunneling process can be expressed by a formula as follows:

    where $\Delta x$ means the length of the tunneling process. We can calculate that the transmission probability is high enough when $\Delta x$ is less than 3.0 nm. The effective barrier height for electrons between ITO and Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ is 0.9 eV with the Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ thickness of 3.0 nm considering that the direct tunneling effect allows for a significant tunneling current when the thickness of the insulating layer is less than 3.0 nm. The voltage on the insulating layer is about 6 V when the thickness of the Ta$_{\mathrm{2}}$O$_{\mathrm{5}}$ layer is 20 nm in the EIS structure. The breakdown threshold voltage should be 8.8 V which is accordant with the test result considering that the conventional V-LED has a threshold voltage of 2.8 V approximately.

    One way to evaluate the effectiveness of current spreading is by comparing the emission uniformity of the EIS-LED with the lateral structure LED (L-LED) and the vertical structure LED (V-LED) under different current densities (2, 5, 10 and 25 A/cm$^{\mathrm{2}})$, which are demonstrated in Fig. 4. The optical intensities at different injection currents are normalized by the CCD camera for comparison. Relative light emission intensities are indicated by the color bar located in the left-hand side of the light emission images, where the white-color zone represents the highest light emission intensity. As shown in Figs.4(a) and 4(b), the intensity of light emission through the electrode area is significantly lower than the area without an electrode, which indicates a poor light extraction of the L-LED and V-LED. Furthermore, the light emission uniformity is uneven between the PN fingers, which is caused by the current crowding phenomenon. The current crowding phenomenon becomes more obvious at higher current density. For the EIS-LED, very uniform emission is obtained when the injection current density ranges from 2 to 25 A/cm$^{\mathrm{2}}$, which can be seen in Fig. 4(c).

    We also perform a one-dimensional power density image to verify the current spreading effect of the EIS structure. As shown in Fig. 5, the cross lines represent the scanning paths on the three types of LEDs and the standard deviation of power density distribution is calculated then. The power density of the L-LED ranges from 2.5 to 5.5 W/cm$^{\mathrm{2}}$ according to the measurement and the V-LED shows an extraordinarily unequal distribution of power density owing to the longer distance between each pair of finger electrodes. However, the EIS-LED performs a uniform distribution property of the light output power density. For the EIS-LED, the standard deviation of power density distribution in light distribution was 0.028 ($s=0.028$), much smaller than that of lateral structure LED ($s=0.78$) and vertical structure LED ($s=1.03$), which illustrated a huge advantage on the current spreading ability of EIS structure. Comparing the structure of EIS-LED with that of V-LED, we can easily find that this result might be due to the effective current spreading from the electrode-insulator-semiconductor (EIS) structure. These results show the enormous potentiality of the EIS structure in order to realize high-power and high-efficiency LEDs with uniformly distributed current injection.

4.   Conclusions
  • To summarize, we demonstrated the use of the electrode-insulator-semiconductor structure as a current spreading layer for the GaN-based LEDs. The $I$-$V$ curves of the EIS-LED show a unique property when the devices switch from the high resistance status (HRS) to the low resistance status (LRS) with the threshold voltage of 3.2 V. The EIS-LED exhibited better emission uniformity compared with the L-LED and the traditional V-LED when the injection current density ranges from 2 to 25 A/cm$^{\mathrm{2}}$, which indicates reduced current crowding as well as better current spreading. The standard deviation of power density distribution in light distribution was much lower than the lateral structure LED and vertical structure LED. The improvement in the current spreading may be attributed to the excellent current injection property of EIS structure.

Figure (5)  Reference (19) Relative (20)

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