The Introduction of HP4156A

2.2.6 The Introduction of HP4156A

HP4156A is an electronic instrument for measuring and analyzing the characteristics of semiconductor devices. This one instrument allows us to perform both measurement and analysis of measurement results.

HP4156A has four highly accurate source/monitor units (SMUs), two voltage source units (VSUs), and two voltage measurement units (VMUs). The HP4156A is designed for Kelvin connections and has high-resolution SMUs (HRSMUs), so HP4156A is especially suited for low resistance and low current measurements. We can measure voltage values with a resolution of 0.2μV by using the differential measurement mode of VMUs.

HP4156A can perform stress testing. That is, can force a specified dc voltage or current for the specified duration.

Also, we can force ac stress by using pulse generator units (PGUs), which are installed in HP41501A SMU/Pulse Generator Expander. The HP41501A is attached to HP4156A, and can be equipped with a ground unit (GNDU), high power SMU (HPSMU), two medium power SMUs (MPSMUs), or two PGUs.

HP4156A can print and store, in addition to performing measurement and analysis. We can store measurement setup information, measurement data, and instrument setting information on a 3.5-inch diskette inserted into the disk drive of HP4156A. And we can print the setting information and measurement results on a plotter or printer that is connected to HP4156A.

HP4156A can be controlled by an external controller via HP-IB by using remote control commands. These commands are based on Standard Commands for

Programmable Instruments (SCPI), so we can easily develop measurement programs.

HP4156A has internal HP instrument BASIC, so we can develop and execute measurement programs by using the HP4156A only, without using an external controller.

Chapter 3

Results and Discussion

3.1 Properties of Films

3.1.1 Sheet Resistance of ITO Film

Figure 3-1 shows the sheet resistance of 180 nm and 300 nm thick ITO films which were evaporated on quartz substrates at two oxygen flow rates. When the flow rate is 30 sccm, we can find the sheet resistance decreasing after annealing at higher temperature. On the other hand, if the oxygen flow rate is 15 sccm, sheet resistance will have a maximum value after 500 annealing℃ . The appearances are similar on different thicknesses of ITO films. It is well known that the oxygen content in ITO films is a critical control parameter of the resistivity of the film. Conduction is partially a result of unfilled oxygen vacancies, so a high incorporation of oxygen into the film will result in few vacancies and a highly resistive film [36]. When annealing at higher temperature, the oxygen atoms may easily evaporate to the air. It will increase the amount of oxygen vacancies and attribute to a better conductivity of ITO film. Figure 3-2 shows the relationship between ITO film thickness and the sheet resistance. By any means, the sheet resistance decreases while increasing the thickness of ITO film due to a higher conductivity, as our preconception. The measured sheet resistances of all ITO films are listed in table 3-1.

3.1.2 Scanning Electron Microscope (SEM) Analysis

Figure 3-3 shows SEM pictures of ITO films with different thicknesses. ITO films were observed to be polycrystalline and showed a similar crystal structure to that of undoped In2O3. The grain size is about 70 nm, 120 nm, and 160 nm for 60 nm, 180 nm, and 300 nm ITO film, respectively. These pictures indicate that the grain size

of the films increased with growing film thickness. In the thin film growth, it is generally observed that the grain size increases with increasing film thickness [37, 38].

The larger grain size results in a lower density of grain boundaries, which behave as traps for free carriers and barriers for carrier transport in the film [39]. Hence, an increase in the grain size can cause a decrease in grain boundary scattering, which leads to an increase in the conductivity. It may be one reason of why thicker ITO film got a smaller sheet resistance.

3.1.3 Transmittance of ITO Film

Figure 3-4 shows the optical transmittance as a function of wavelength of the evaporated ITO film on GaN substrate with different annealing temperatures. In this figure, the transmittance of each film was directly compared with air. By varying the annealing temperature, the transmittances are almost the same. However, the curves were long-wavelength shifted after annealed at higher temperature. It means an increase of the refractive factor of ITO film. The transmittance of different ITO film thicknesses are showed in Fig. 3-5. For comparison, the most commonly used Ni(4 nm)-Au(4 nm) contact layer that was annealed at 540℃ in nitrogen ambience was also examined. The transmittances are about 80% at a wavelength of 465 nm of all thicknesses of ITO films. Compared with Ni/Au film, transmittance of ITO has an enhancement of about 20%.

3.2 Interfacial Characteristics

3.2.1 Specific Contact Resistance of ITO or Ni/Au Layer on p-GaN

Figure 3-6 shows the current-voltage (I-V) characteristics of Ni/Au (4 nm/4 nm) and ITO (300 nm) contacts on p-GaN after annealing at temperatures of 400-600 ℃ for 10 minutes in N2 ambient. It was found that both ITO and Ni/Au films could form

good ohmic contacts on p-GaN. Specific contact resistance was determined from plots of the measured total resistance versus the spacings between the TLM pads. The least square curve-fitting method was used to fit a straight line to the experimental data, as shown in Fig. 3-7. Although ITO and Ni/Au films both have ohmic contacts on p-GaN, Ni/Au still has the smaller specific contact resistance than all ITO films. To explain the low resistance of Ni/Au contacts, Ho et al. proposed that NiOx layer on the p-GaN epilayer surface causes a reduction of the schottky barrier height across the interface [40]. However, Maeda et al. suggested that oxygen reacts with hydrogen in the Mg-H bonds and, thus, reduces the hydrogen concentration and modifies the energy band structure in the interface region [41]. Moreover, another study suggested that ohmic contact characteristics result from the removal of surface contamination on GaN by Ni before or during layer reversal [42]. Figure 3-8 exhibits the specific contact resistances of ITO films on p-GaN which were evaporated with different oxygen flow rates and annealed at 400-600 .℃ It is obvious that the specific contact resistance had a minimum value at 500℃ with 15 sccm O2 flow rate and increased with the annealing temperature with 30 sccm O2 flow rate. This may be due to the variance of oxygen proportion in the ITO/p-GaN interface which will combine with gallium to form GaO. Moreover, ITO films that were evaporated with 30 sccm oxygen flow rate have a higher specific contact resistance than with 15 sccm. This may be due to the incorporation of oxygen. The relationship between specific contact resistance and ITO film thickness is not discussed because the thickness should not affect the interface characteristics. A particular data of all samples’ specific contact resistance is listed in table 3-2. The sheet resistance of p-GaN was also calculated simultaneously and is listed in table 3-3. The fluid values of p-GaN’s sheet resistance may be attributed to the instability of carrier concentration. However, the sheet resistance of p-GaN is larger than that of ITO film and result in a poor conductivity. That’s why we need an

additional transparent conducting layer upon p-GaN.

3.2.2 Current Spreading Behavior by Calculation

After the sheet resistances and specific contact resistances were known, we started the current spreading calculation. The calculated current distribution of the LED is shown in Fig. 3-9. As the results by calculation, we can conclude that thicker ITO film contributes to a uniform current spreading phenomenon. This instance is chiefly caused by the smaller sheet resistance of thicker film. It can be seen that current crowding can be alleviated via the use of a thicker ITO film. For simple expression, we define current spreading length Ls as the length where the current density has dropped to the 1/e value of the current density at the p-pad. Figure 3-10 shows the calculated current spreading length of different thicknesses of ITO films that were evaporated with 15 sccm oxygen flow rate. This figure indicates that no matter at what annealing temperature, current will spread farther by increasing ITO film’s thickness due to the lower sheet resistance. Figure 3-11 shows Ls of 300 nm ITO films which were evaporated with different oxygen flow rates. With a 30 sccm oxygen flow rate during evaporation, sample annealed at 600℃ has a longest Ls compared with other temperatures because of its largest specific contact resistance.

The large specific contact resistance makes the current tend to flow horizontally.

Same theory can also explain the shortest Ls at 500 with 15 s℃ ccm oxygen flow rate by having the smallest specific contact resistance. Ls of ITO samples are listed together in table 3-4.

3.3 Electrical and Optical Properties of Devices 3.3.1 Reverse Current-Voltage Characteristic of LEDs

Not only forward voltage was applied, reverse voltage was also used to check the

value of leakage current, as shown in Fig. 3-12. The leakage current is only several nA which is small enough as our toleration, and not affected by annealing temperature.

The fabricated LEDs’ leakage currents are generally small enough except two wafers.

Figure 3-13 points out the high leakage current of these devices with 180 nm ITO film evaporated with 15 sccm oxygen flow rate and annealed at 400℃. It may be caused by the bad epitaxy property of GaN wafer and will result in the output power’s degradation and a serious thermal effect.

3.3.2 Forward Current-Voltage Characteristic of LEDs

Figure 3-14 depicts the room-temperature forward current-voltage characteristics of the fabricated GaN-based LEDs. These samples were all annealed at 400 .℃ In Fig.

3-14(a), the 20-mA forward voltage measured from LEDs with 60 nm ITO, 180 nm ITO, 300 nm ITO, and Ni/Au on p-GaN was 3.45, 3.41, 3.32, and 3.14 V, respectively.

Apropos of LEDs with films which were evaporated with high oxygen flow rate, the forward voltage was 3.5, 3.31, 3.26, and 3.08 V, respectively, as shown in Fig. 3-14(b).

The forward voltage decreases with an increase of ITO film’s thickness. As the specific contact resistance doesn’t form an order, the forward voltage may be affected by sheet resistance, or conductivity of TCL. We can read from the figure that Ni/Au film still has the smallest Vf due to its corking conductivity. Figure 3-15 shows the comparison of forward voltage with different annealing temperature. In Fig. 3-15(a), the 400℃-annealed sample’s large forward voltage is attributed to the large specific contact resistance. Similarly, in Fig. 3-15(b), the forward voltage is also affected by the specific contact resistance.

3.3.3 Dynamic Resistance and Series Resistance of LEDs

Figure 3-16 shows forward voltage versus dynamic resistance (dV/dI) in LEDs

with different thicknesses of ITO films annealed at 400 .℃ The dynamic resistance decreases while the voltage increases because more and more free electron-hole pairs are generated. Values at the tail of the curves are close to their own series resistance.

It can be seen that the series resistance is in association with the thickness of ITO film.

The approximated series resistances are listed in table 3-5. All series resistances have a decrease with thicker ITO films. While the ITO’s thickness is fixed, annealing temperature almost has no influence on the series resistance. Thus, we can conclude that the series resistance is mainly dominated by sheet resistance. Hence, Ni/Au film’s small series resistance is reasonable due to its good current spreading ability.

3.3.4 Luminance-Current Characteristic of LEDs

Figure 3-17 presents the normal light output as a function of injection current for fabricated LEDs with 300 nm ITO film annealed at different temperatures. Devices with ITO film are all brighter than that with Ni/Au film. Figure 3-18 shows the luminance enhancement as a function of injection current for LEDs with different thicknesses of ITO films. The ITO samples were compared and normalized with Ni/Au samples which were cut from the same wafer to ensure a reliable result. We could achieve a factor of about 1.3-1.5 times luminance enhancement by the difference of optical transmittance between ITO and Ni/Au films. Notice that the enhancement of devices with 60 nm ITO film has a decrease when the injection current increasing. On the other hand, devices with 300 nm ITO film don’t have such a decrease. It is supposed that because the thin ITO film has a heavier thermal effect than thick ITO film and Ni/Au film. The heavy thermal effect is attributed to the poor conductivity which forms a large resistance. Figure 3-19 presents the luminance enhancement with different ITO films at an injection current of 20-100 mA. No matter what the evaporating oxygen flow rate is, the thermal effect is serious with thin

(60 nm) ITO films while thick (300 nm) ITO films almost remain the enhancement factor in constant. Normal luminance of all ITO samples at different injection current is shown in Fig. 3-20. From Fig. 3-13, we have known that the leakage current in condition B is extremely high, especially in the 400 annealing one.℃ It will cause the degradation of output light, as shown in Fig. 3-21. The high density of black dots in Fig. 3-21(a) is caused by the high leakage current. Hence, the circled data in Fig. 3-20 may not be accurate. In addition, it seems that the luminance of each device doesn’t form a regular relationship. Figure 3-22 shows the relationship between calculated current spreading length and forward voltage or normal luminance. From Fig. 3-22(a), we find that LS decreases while the forward voltage increasing, where the straight line is an approximation line fitted by least square method. This may be resulted by the specific contact resistance. A large rC will cause a long current spreading length and a small 20-mA forward voltage. However, in Fig. 3-22(b), it seems that the optical performance is not corresponding to current spreading length. A possible reason is that most of the current spreading lengths are larger than the device size (300 × 300 μm), so almost all devices have a uniform enough current distribution. Figure 3-23 shows the current converting efficiency (dL/dI) of LEDs as a function of injection current. When the injection current becomes larger, a decrease of the converting efficiency could be discovered. It may be a reason of why the luminance has no relationship with LS. Although thicker ITO film has longer current spreading length, the increment in light output is not as much as in the current density. Then, such declined light enhancement may be disrupted because the thickness of evaporated ITO film might not be exact enough as our settings.

Chapter 4 Conclusion

In this experiment, we have fabricated GaN-based LEDs with different thicknesses of ITO films and also found the optical and electrical characteristics of them. First, we observe that the sheet resistance of ITO films on quartz has a decrease when the films grow thicker, which indicates a better conductivity. However, the sheet resistance is also dominated by the annealing temperature due to the change of the amount of oxygen vacancies and grain size of ITO film. From SEM pictures, we can discover that the grain size of ITO film has an obvious increase with its thickness.

Next, the transmittances of these ITO films are all about 80% at a wavelength of 465 nm and have 20% larger than Ni/Au films.

Moreover, the specific contact resistance increases with the annealing temperature, which is attributed to a thin GaO layer formed in the ITO/p-GaN interface. The calculated current distribution and current spreading length are affected by sheet resistance and specific contact resistance, especially the sheet resistance. A small sheet resistance, or a thick ITO film, demonstrates a good current spreading ability. Some of the calculated current spreading lengths are even larger than the size of LED device. The thickness of ITO film is also corresponding to the operating voltage due to the variance of conductivity. However, the film thickness seems no relationship with the normal light output. Compared with the references, all LEDs with different ITO films have a factor of about 1.3-1.5 times luminance enhancement by the difference of optical transmittance. By generalizing the above results, the best thickness of ITO film can be 300 nm.

Chapter 5 Future Work

In the future work, the similar experiment for other TCL structures with an intermediate metal layer, such as Ni/ITO and Ag/ITO, will be carried out. Larger size LEDs can even be fabricated because the current spreading length of a thick ITO film can be in excess of 1000 μm. In addition, after finding the best thickness of ITO film, it will be combined with surface roughening technique to achieve higher light extraction efficiency. This may help fabricating LEDs with an additional luminance enhancement.

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